Q  E 

12.0 


UC-NRLF 


B   M   512  fl?D 


x  r 


LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


GIFT  OF 


^  \ 

Class 


THE  GEOLOGY 


OF  THE 


FOX  ISLANDS,  MAINE, 


A  Contribution  to  the  Study  of  Old  Volcanics. 


A  DISSERTATION  SUBMITTED  TO  THE  BOARD  OF 

UNIVERSITY  STUDIES  OF  THE  JOHNS  HOPKINS  UNIVERSITY  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY. 


BY 


GEORGE  OTIS  SMITH. 


THE  GEOLOGY 


OF  THE 


FOX  ISLANDS,  MAINE, 


A  Contribution  to  the  Study  of  Old  Volcanics. 


A  DISSERTATION  SUBMITTED  TO  THE  BOARD  OF 

UNIVERSITY  STUDIES  OF   THE  JOHNS  HOPKINS  UNIVERSITY  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY. 


BY 


GEORGE  OTIS  SMITH 

OF  THE 

UNIVERSITY 

FO 

•• 

PUBLISHED  BY  THE  AUTHOR, 
SKOWJ4EGAN,  MAINE. 


Q 


PRESS     OF 

J.  0.  SMITH  &  Co. 

SKOWHEGAN,    ME. 


CONTENTS. 


PAGE 

INTRODUCTORY: 

Geographical,  ' 

Topographical,  7 

Geological, 
Nature  of  Problem, 

Areal  Distribution  of  Rock-types,  11 

NORTH  HAVEN  GREENSTONE  COMPLEX: 
Distribution, 

Description  of  Rock-types, 
Diabases, 
Amyg-daloids, 

Tuffs,       -  IS 

Columnar  Structure,       -  16 

Age,  19 

NIAGARA  SEDIMENTS: 
Distribution, 

Southern  Harbor  Section,  20 

Fauna, 

Conditions  of  Deposition, 
CALDERWOOD'S  NECK  SCHISTS: 
Description  and  Distribution, 
Relations, 

Age,  -  -       - 29 

THOROUGHFARE  VOLCANICS: 
Distribution, 

Description  of  Rock-types, 
Andesites, 
Porphyries, 
Amygdaloids,     -  36 


CONTENTS. 

Pyroclastics,  37 

Sequence,        -  40 

Ag-e,  42 

Conditions  of  Volcanic  Activity,  43 

VINAL  HAVEN:  ACID  VOLCANICS: 

Distribution,      -  46 

Description  of  Rock-types,  46 

Aporhyolites,  46 

Flow-breccias,  51 

Tuffs,    ..*  51 

Sequence,       -  52 

Conditions  of  Volcanic  Activity,       -  52 

DIKES: 

Description,  -    56 

Distribution,      -  57 

DIABASIC  AND  GRANITIC  INTRUSIVES: 

Areal  Distribution,  59 

Granite,  60 

Diabase  and  Diorite,      -  61 

Relative  Ag-e,  63 

Relations  to  Dikes,  64 

Time  and  Conditions  of  Intrusion,  64 

ALTERATION  : 

Character  of  Alteration,  66 

Structural  and  Mineralog-ical,     -  66 

Devitrification,  69 

Nomenclature  of  Altered  Ig-neous  Rocks,  70 

GEOLOGIC  HISTORY: 

Acadian  Troug-h,  72 

Pre-Niag-ara  Volcanic  Activity,  72 

Niag-ara  Sedimentation,  73 

Post-Niag-ara  Volcanic  Activity,      ,-  74 

Devonian  Intrusions,  75 

PLATES. 

I.  Photo-micrographs. 

II.  Geolog-ical  Map. 


PREFACE. 

The  following"  geological  study  of  the  Fox  Islands  is  presented 
as  a  contribution  to  th^  solution  of  the  problem  of  the  identifica- 
tion and  interpretation  of  ancient  volcanic  rocks.  The  writer 
owes  his  interest  in  this  subject  to  the  late  Professor  Georg-e  H. 
Williams,  whose  enthusiasm  in  petrographical  research  was  so  in- 
spiring* to  his  students. 

The  Fox  Islands  area  was  visited  and  studied  in  1893  by  Dr.  W. 
S.  Bayley  of  Colby  University,  and  the  writer  wishes  to  acknowl- 
edge the  kindness  of  Dr.  Bayley  in  granting-  him  the  privilege  of 
studying"  the  rocks  collected  at  that  time,  as  well  as  the  accom- 
panying- full  suite  of  thin-sections.  The  area  was  studied  and  the 
different  series  mapped  by  the  writer  in  the  spring-  of  1895,  and  a 
subsequent  visit  was  made  in  the  spring-  of  1896. 

In  the  preparation  of  this  paper,  assistance  has  been  received 
from  Prof.  Clark,  Dr.  Mathews,  and  Messrs.  Gilbert  and  Willis  of 
the  Geological  Department  of  this  University,  and  Dr.  Bayley  of 
Colby  University,  to  all  of  whom  the  writer  desires  to  express  his 
indebtedness.  He  also  wishes  to  acknowledge  the  courtesy  of  Pro- 
fessors Remsen  and  Morse  of  the  Chemical  Department,  under 
whose  direction  rock-analyses  were  kindly  made  by  Messrs.  K.  W. 
Mag-ruder  and  W.  A.  Jones. 


INTRODUCTORY. 

GEOGRAPHICAL.  The  rock-bound  coast  of  Maine  offers  special 
attractions  to  the  geologist.  It  is  characterized  by  many  deep  in- 
dentations, the  largest  being  Penobscot  Bay,  in  which  are  situated 
the  Fox  Islands.  These  islands,  a  geological  study  of  which  is 
here  presented,  are  slightly  over  eight  miles  due  east  from  the  city 
of  Rockland  and  about  twenty-five  southwest  from  Mount  Desert. 
Like  the  latter  island,  the  Fox  Islands  lie  just  within  the  general 
coast-line  which  has  a  southwest  and  northeast  trend.  Unlike 
Mount  Desert,  however,  they  are  at  some  distance  from  the  main- 
land, being  nearly  in  the  middle  of  the  bay,  which  is  twenty-eight 
miles  wide,  from  Owls  Head  to  Naskeag  Point.  To  the  southeast 
and  southwest,  these  islands  are  exposed  to  the  open  Atlantic,  but 
to  the  south  about  ten  miles  distant  is  an  outer  fringe  of  small 
islands,  the  Matinicus  group. 

North  Haven  and  Vinal  Haven,  the  principal  members  of  the 
Fox  Islands,  are  among  the  larger  of  the  many  rocky  islands  on 
the  Maine  coast.  Surrounding  these  are  many  smaller  islands, 
and  the  group  extends  ten  and  three-fourths  miles  from  north  to 
south  and  eight  and  one-fourth  from  east  to  west1. 

TOPOGRAPHICAL.  Topographically,  these  islands  are  well  di- 
versified. Although  at  no  point  is  there  any  great  elevation,  yet 
many  hills  rise  from  one  to  two  hundred  feet  above  the  sea-level, 
and  afford  pleasing  variety  of  relief.  The  most  striking  topo- 
graphical feature,  however,  is  the  extremely  irregular  coast-line, 
which  is  to  be  regarded  simply  as  an  unmodified  contour  of  the 
rocky  relief.  Peninsulas,  necks  and  points,  bays,  harbors  and 
coves  are  the  elements,  which  constitute  the  greatest  charm  of  the 
Fox  Islands.  Although  one  might  sail  around  the  islands  in  a  dis- 

i.  Maps  of  the  Fox  Islands  are  sheets  309  and  310,  East  and  West  Penobscot  Bay 
scale  i  140,000,  and  31  la,  Fox  Islands  Thoroughfare,  I  : 20,000,  of  the  U.  S.  Coast  and 
Qeodetic  Survey.  Sheets  309  and  310  were  used  to  furnish  the  base  of  the  geological 
map  herewith  presented,  Plate  II. 


8  GEOLOGY   OF   THE   FOX    ISLANDS. 

tance  of  thirty  miles,  the  coast-line  of  the  two  larger  islands  alone 
measures  nearly  one  hundred  and  fifty  miles.  Indeed,  so  deeply 
indented  are  the  shores,  and  so  low  the  divides,  that  a  submerg-- 
ence  of  less  than  twenty  feet  would  divide  North  Haven  and  Vinal 
Haven  into  scores  of  smaller  islands. 

In  connection  with  the  present  topography  it  is  of  interest  to 
note  its  origin,  as  well  as  the  [modifications,  which  are  at  present 
being-  made  in  it.  Both  the  surface  features  of  these  islands  and 
the  deeply  incised  coast-line  have  had  their  origin  in  the  erosion 
processes  of  the  past.  Penobscot  Bay  is  one  of  the  many  drowned 
river  valleys  on  the  New  England  coast;  and  the  islands  which 
now  comprise  so  large  a  part  of  the  area  between  the  shores  of  the 
Bay,  simply  represent  the  tops  of  the  hills  in  the  old  Penobscot 
valley.  Before  the  depression  of  the  land,  the  time  of  which  Prof. 
Shaler,  in  his  study  of  the  geology  of  Mount  Desert,  has  fixed  as 
about  the  close  of  the  Glacial  period,  what  we  may  term  the  Fox 
Hills  rose  within  the  valley  of  the  Penobscot  just  as  the  more 
prominent  Camden  Hills  still  rise  on  the  western  side  of  the  Bay. 
The  Penobscot,  which  now  reaches  tide  water  forty  miles  to  the 
north,  then  had  its  mouth  perhaps  as  many  miles  to  the  south. 

The  Thoroughfare,  now  separating-  the  two  islands,  was  then  a 
transverse  valley  extending-  down  on  either  side  to  the  floor  of  the 
larg-er  valley  and  occupied  by  side  streams.  In  a  similar  way,  the 
harbors  and  coves  mark  the  drainag-e  lines  of  the  old  Fox  Hills. 
Such  topography  naturally  expresses  the  different  degrees  of  rock 
resistance  to  erosion,  and  the  best  example  of  this  is  the  cutting-  of 
the  softer  sedimentary  rocks  on  North  Haven  almost  down  to  what 
is  now  sea-level,  while  the  dense  volcanic  rocks  immediately  to  the 
south  rise  abruptly  as  Ames  Knob.  This  topography,  then,  was 
the  product  of  atmospheric  ag-encies,  and  differs  in  no  essential  re- 
spect from  that  which  is  being-  produced  to-day  under  similar  con- 
ditions. 

During-  the  ice  invasion  of  Glacial  time,  some  modification  of 
the  older  topography  may  have  resulted.  It  is  to  be  noted,  how- 
ever, that  the  characteristic  features  of  Mount  Desert,  interpreted 
by  Shaler  and  Davis  as  due  to  glacial  erosion,  are  not  represented 
on  these  islands.  The  more  prominent  inlets,  especially  the 
Thoroug-hfare,  might  be  compared  to  the  remarkable  transverse 


INTRODUCTORY.  9 

valleys  of  the  Mount  Desert  rang-e,  except  that  the  former  have  a 
g-eneral  trend  more  nearly  east-west,  while  the  glacial  striae  show 
a  north-south  direction. 

The  mantle  of  g-lacial  drift,  consisting-  of  clay  and  sand,  with 
some  gravel,  is  comparatively  thin  on  the  Fox  Islands,  yet  drain- 
ag-e  may  have  been  embarassed  somewhat  by  local  deposits. 
Fresh  Pond  on  North  Haven,  originated,  doubtless,  in  the  dam- 
ming-, at  that  point,  of  a  stream  with  its  several  tributaries.  In 
the  main,  however,  the  pre-Glacial  topography  was  only  slig-htly 
modified  by  the  ice-cap  which  covered  the  Fox  Hills. 

Just  as  the  topography  of  the  present  has  had  its  origin  in  the 
past,  so  that  of  the  future  is  being-  determined  by  the  ag-encies  and 
processes  of  to-day.  The  complex  succession  of  inlets  and  pro- 
montories sig-nifies  a  young-  coast-line.  With  lapse  of  time,  as 
maturity  and  old  ag-e  are  attained,  such  a  coast-line  would  become 
simpler  throug-h  the  action  of  the  waves.  Continued  erosion,  in  the 
same  way,  tends  to  reduce  the  relief  of  the  land;  and  it  is  on  the 
shore  of  these  islands,  that  the  beginning's  can  be  noted  of  the 
chang-e  from  the  present  to  the  future  topographical  features  of 
the  Fox  Islands. 

This  work  in  which  the  waves  are  eng-ag-ed  is  simply  that  of  cut- 
ting- back  the  salient  and  filling-  in  the  re-entrant  features  of  the 
coast.  The  former  process  can  be  observed  at  every  exposed  point 
on  the  islands,  and  the  latter  is  no  less  apparent  in  many  of  the 
coves  between  these  points.  Thus,  at  Oak  Hill,  the  extreme 
northern  point  on  North  Haven,  the  one  process  is  to  be  seen, 
while  on  either  side,  beaches  and  sea-walls  of  shingle  and  sand 
have  been  thrown  across  the  coves  and  the  streams  dammed,  so 
that  marshes  now  mark  what  was  once  the  upper  part  of  the  coves. 
In  many  localities  similar  relations  can  be  observed,  and  indeed  it 
may  be  possible  that  on  the  long-  southwestern  peninsula  of  North 
Haven,  barrier  beaches  of  this  character  may  have  connected  an 
old  smaller  island,  about  a  half  mile  long-,  to  the  larg-e  island, 
forming-  what  is  Crabtree  Point. 

Another  constructive  process  allied  to  the  one  just  mentioned  is 
that  of  sedimentation  at  the  upper  part  of  the  long-  coves  and 
harbors.  The  fine  sediments  are  broug-ht  in  by  the  entering- 
streams  and  by  the  tides  from  other  parts  of  the  shore,  while  the 


10  GEOLOGY   OF   THE   FOX   ISLANDS. 

outflowing-  tides  are  not  strong-  enough  to  sweep  out  the  mud  and 
sand,  except  in  the  channels  where  the  scouring-  action  is  more 
complete. 

Thus,  observation  of  what  is  being-  done  on  the  coast  at  present 
warrants  the  assertion  that  a  continuance  of  these  processes  must 
result  finally  in  a  coast  line  which  will  g-ain  simplicity,  but  at  the 
same  time  lose  its  present  charm  of  variety. 

GEOLOGICAL.  An  oceanic  island  may  be  treated  as  a  geologic 
unit;  but  lying-  as  they  do  within  the  g-eneral  coast-line,  the  Fox 
Islands  are  to  be  reg-arded  simply  as  detached  portions  of  the  main- 
land. Geologically,  these  islands  are  to  be  connected  with  the 
other  areas  of  the  region,  just  as  if  they  were  yet  what  we  have 
termed  the  Fox  Hills.  Correlation  with  the  rocks  of  other  parts  of 
this  coast  is  necessary  to  a  full  understanding  of  the  geologic 

problem. 

It  is,  therefore,  unfortunate  that  the  amount  of  geological  work 
done  on  the  coast  of  Maine  has  not  been  greater.  The  earlier 
State  surveys  of  Jackson1  and  of  Hitchcock2  were  only  reconnais- 
sances of  the  mineral  and  agricultural  resources.  Later  works  by 
Professors  Shaler3,  Davis4,  and  Crosby5,  however,  furnish  detail 
studies  of  limited  areas  to  the  east  of  Penobscot  Bay,  while  that  of 
Dodge  and  Beecher6  is  invaluable  as  determining  the  age  of  the 
sedimentary  series  on  North  Haven,  and  calling  attention  to  the 
presence  of  associated  volcanic  rocks.  The  volcanic  areas  on  the 
Maine  coast  were  also  mentioned  by  the  late  Professor  Williams  in 
his  inspiring  essay  on  the  volcanic  rocks  of  eastern  North  Ameri- 
ca7; and  the  importance  of  the  volcanic  series  of  the  Fox  Islands 
has  been  noted  by  Dr.  W.  S.  Bayley8  and  the  writer9. 


1.  Annual  Reports  on  the  Geology  of  the  State  of  Maine,  1837,  l83§,  1839. 

2.  Reports  on  the  Geology  of  Maine,  1861,  1862. 

3.  Geology  of  Mount  Desert.     8th.  Annual  Report,  U.  S.  G.  S. 
Am.  Jour,  of  Science  (3rd  ser.)  Vol.  32,  p.  35. 

4.  An  Outline  of  the  Geology  of  Mt.  Desert.     Introd.   to  Flora  of  Mount   Desert  Isl. 
Me.     1894. 

5.  Proc.  Boston  Soc.  Nat.    Hist.     1880.  p.  109.     Am.  Jour,  of  Science  (3rd  ser.)  Vol. 
23,  p.  64.      1882. 

6.  Am.  Jour,  of  Science  (3rd  ser.)  vol.  43,^).  412.  1892. 

7.  Journal  of  Geology,  vol.  2,  p.  i.  1894. 

8.  Bull.  Geol.  Soc.  Am.,  vol.  6,  p.  474.   1894. 

9.  Johns  Hopkins  Univ,  Circular  No.  121.  1895. 


INTRODUCTORY.  11 

In  its  broader  geologic  relations,  this  group  of  islands  is  to  be 
considered  as  forming  a  part  of  the  Acadian  province,  which  has 
been  recognized  as  an  area  of  sedimentation  throughout  Paleozoic 
time;  and  the  position  of  the  Fox  Islands  on  the  northern  side  of 
this  great  trough  must  be  kept  in  mind  as  the  geologic  history  is 
traced  out.  The  work  of  the  Canadian  geologists  may  assist, 
since  the  periods  of  sedimentation  and  of  disturbance  were  pro- 
bably the  same  throughout  the  whole  geological  province. 

NATURE  OF  THE  PROBLEM.  The  problem  is  three-fold:  To  in- 
vestigate the  geological  relations  of  the  different  members  of  the 
rock  series  on  the  Fox  Islands;  to  determine  the  original  nature  of 
the  rock-types  represented;  and  to  trace  the  changes  which  they 
have  subsequently  undergone.  The  study  is,  thus,  both  geotec- 
tonic  and  petrographic.  Its  object  is  to  obtain  such  data  as  to 
suggest  an  outline,  at  least,  of  the  geologic  history  of  the  Fox 
Islands,  and,  perhaps,  to  contribute  to  the  determination  of  those 
conditions  which  prevailed  in  the  Penobscot  Bay  area  at  a  time 
when  it  was  the  scene  of  long  continued  and  widely  extended 
volcanic  action. 

AREAL  DISTRIBUTION  OF  ROCK-TYPES.  In  their  whole  extent, 
the  two  islands  of  North  Haven  and  Vinal  Haven  present  consider- 
able variety  in  the  rocks  which  compose  them.  The  bold  character  of 
the  coast  and  its  great  extent,  due  to  the  many  indentations,  afford 
opportunity  to  realize  the  complexity  of  these  rock  masses.  In 
this  complexity,  however,  there  is  system,  and  in  the  areal  distri- 
bution of  the  rock-types  represented  is  to  be  found  the  first  clue  to 
the  solution  of  the  geologic  problem  which  is  the  subject  of  the 
present  study.  The  delimitation  of  the  different  areas  must  be 
preparatory  to  the  study  of  the  rock-types  and  their  relations. 

The  island  of  North  Haven  is  composed  of  greenstone  schists, 
with  the  exception  of  the  two  peninsulas  which  extend  southwest 
into  the  Thoroughfare,  Indian  Point  and  the  larger  one  between 
Waterman's  Cove  and  Southern  Harbor.  Since  these  schists  pre- 
sent considerable  variation,  the  area  may  be  termed  the  North 
Haven  Greenstone  Complex. 

Immediately  south  of  the  green  schists,  and  extending  eastward 
from  the  shores  of  Southern  Harbor,  is  a  narrow  strip  of  sand- 


12  GEOLOGY   OF   THE    FOX   ISLANDS. 

stones,  shales,  conglomerates,  and  limestones,  a  fossiliferous  series 
of  Niagara  Sediments. 

Other  sedimentary  rocks,  quartzitic  slates,  wholly  different  from 
those  of  Niagara  age,  are  found  on  the  northeastern  part  of  Vinal 
Haven;  these  will  be  called  the  CalderwoocTs  Neck  Series. 

Occupying  the  greater  part  of  the  two  southern  peninsulas  of 
North  Haven,  already  mentioned,  are  the  red,  gray  and  purple 
volcanic  rocks.  These  continue  across  the  Thoroughfare  to  the 
northeastern  shore  of  Vinal  Haven  and  the  northwestern  part  of 
Calderwood's  Neck,  and  the  series  being  typically  developed  on 
the  shores  of  the  Thoroughfare  may  be  termed  the  Thoroughfare 
Volcanics. 

The  northwestern  part  of  Vinal  Haven,  with  the  exception  of 
the  strip  of  volcanics  just  mentioned,  is  characterized  by  a  differ- 
ent series  of  volcanic  rocks,  more  acid  in  composition.  These  will 
be  described  as  the  Vinal  Haven  Acid  Volcanics. 

Throughout  these  various  areas  are  found  dikes  of  igneous  rocks, 
both  acid  and  basic,  which  deserve  mention. 

.  The  southern  and  greater  part  of  the  island  of  Vinal  Haven  is 
made  up  of  holocrystalline  rocks,  granite,  diabase,  and  diorite,  the 
Intrusives. 

In  brief,  such  are  the  areas,  which  will  be  considered  in  the  fol- 
lowing pages.  In  the  main,  this  order  of  discussion  of  the  areas  is 
that  from  north  to  south,  a  succession,  however,  which  will  be 
seen  to  be  geological  as  well  as  geographical. 


NORTH  HAVEN  GREENSTONE  COMPLEX. 

DISTRIBUTION.  The  greenstone  area,  as  already  denned,  includes 
the  greater  part  of  North  Haven.  Of  this  triangular  island, 
the  northwestern  coast,  almost  all  of  the  eastern,  and  the  greater 
part  of  the  southern  are  composed  of  greenstone  schists,  which  are 
also  found  outcropping-  on  the  low  hills  of  the  interior.  The  topog- 
raphy of  the  region  is  without  conspicuous  features  and  the 
northern  coast-line  is  rather  exceptional  in  its  simplicity.  The 
general  trend  of  the  hills  seems  to  be  determined  by  that  of  the 
rock  structure  or  by  intrusive  dikes.  Of  the  whole  area,  that  por- 
tion bordering-  upon  Southern  Harbor  presents  the  most 
variety  in  rock-types  and  structures.  Here  also,  the  relations  of 
the  greenstones  to  the  rocks  which  lie  to  the  south  can  be  observ- 
ed, and  a  more  careful  study  was  made,  therefore,  of  the  rocks  of 
this  portion  of  the  greenstone  area. 

DESCRIPTION  OF  ROCK-TYPES.  The  old  field  term,  "green- 
stone," has  been  employed  for  this  area,  since,  although  not  exact, 
it  is  descriptive.  The  metamorphosed  character  of  the  rocks  is 
evident,  and  the  difficulties  attending-  the  determination  of  their 
original  nature  justify  the  use  of  the  name.  The  greenstones,  in 
the  main,  are  aphanitic  and  of  a  light  yellow  to  grayish  green 
color.  In  portions  of  the  area  the  rocks  are  quite  schistose,  while 
more  generally  they  show  a  columnar  structure  which  will  l?e  con- 
sidered in  a  following-  paragraph. 

Study  of  the  shores  of  Southern  Harbor  shows  the  presence  of 
large  masses  of  amygdaloidal  greenstone,  as  well  as  smaller 
amounts  of  a  rock,  which  is  rather  tuffaceous  in  texture  and  at  one 
point  even  breccia-like.  The  greenstone  is  locally  also  quite  plain- 
ly crystalline,  and  it  is  often  difficult  to  distinguish  between  this 
greenstone  complex,  to  be  regarded  as  a  geologic  unit,  and  the 
later  intrusives.  The  number  of  dikes,  both  acid  and  basic,  is 
greater  here  than  in  any  other  equal  area  on  the  two  islands. 

13 


14  GEOLOGY   OF  THE   FOX   ISLANDS. 

DIABASES.  The  compact  aphanitic  rock  found  just  west  of  the 
head  of  Southern  Harbor  is  the  least  altered  of  the  specimens  ex- 
amined microscopically  and  thus  favors  the  determination  of  the 
original  nature  of  these  greenstones.  The  structure  is  plainly 
diabasic  with  feldspar  laths  mostly  small,  but  often  approaching-  a 
porphyritic  development.  Between  the  feldspar  individuals, 
which  have  a  composition  about  that  of  andesine,  there  are  wedges 
of  chlorite,  nearly  isotropic,  which  may  represent  the  pyroxene 
constituent  or  possibly  some  amount  of  glassy  base.  The  feld- 
spars show  a  considerable  amount  of  alteration  both  at  their  edges 
and  in  the  interior  of  the  larger  laths.  Secondary  minerals  con- 
stitute a  large  part  of  the  rock  section  and  include  clear  grains  of 
zoisite,  small  amounts  of  pyrite  and  calcite,  spherulitic  areas  of 
chlorite,  with  stronger  double  refraction  than  the  other  variety, 
and  water-clear  grains  of  secondary  feldspar,  which  appear  in 
marked  contrast  to  the  cloudy  original  feldspar.  Another  second- 
ary mineral  even  more  abundant  than  the  zoisite  occurs  in  very 
small  yellowish  grains,  highly  refractive  and  appearing  quite 
white  in  incident  light.  The  bright  interference  colors  and  im- 
perfect extinction  make  it  probable  that  this  is  either  epidote  or 
titanite,  very  finely  granular.  Comparison  with  masses  of  the 
latter  mineral  resulting  from  alteration  of  ilmenite,  favors  the  de- 
termination as  titanite.  Some  of  the  sections  contain  considerable 
quantities  of  fine  needles  .005  to  .01  mm.  in  length,  golden  yellow 
in  color,  with  high  refractive  index,  strong  double  refraction, 
parallel  extinction,  and  positive  optical  character,  all  of  which 
characters  belong  to  rutile,  thus  showing  the  presence  of  titanium 
in  these  greenstones. 

The  texture  of  these  rocks  and  their  composition,  as  shown  by 
the  remnants  of  original  minerals  and  the  nature  of  the  secondary 
minerals,  point  to  their  original  nature  having  been  that  of  fine 
grained  diabases  or  possibly  diabase  porphyries. 

The  porphyritic  phase  of  these  diabases  occurs  at  Websters 
Head  on  the  northern  side  of  the  island,  where  in  a  rock,  similar 
to  the  Southern  Harbor  greenstone  in  megascopic  appearance,  the 
diabase  texture  is  less  evident,  the  feldspar  being  more  prominent 
and  in  larger  individuals.  The  rock  has  been  subjected  to  crush- 
ing and  the  feldspar  fractured,  one  twinned  individual  having  its 


NORTH  HAVEN  GREENSTONE  COMPLEX.  15 

fragments  separated  by  quite  wide  intervals.  The  light  green 
chlorite  shows  the  Berlin-blue  interference  tint,  characteristic  of 
pennine.  Brightly  polarizing  fibers  of  a  light  green  mineral  also 
occur  in  connection  with  the  altered  feldspar,  and  from  its 
pleochroism  and  optical  orientation,  this  mineral  is  believed  to  be 
an  amphibole,  probably  actinolite. 

AMYGDALOIDS.  The  amygdaloidal  type  occupies  the  more  south- 
ern parts  of  the  area,  immediately  north  of  the  Niagara  sediments. 
On  the  weathered  surface,  the  grayish  greenstone  is  pitted,  while 
a  fresh  fracture  is  spotted  with  shining  calcite  grains.  In  the  thin 
section,  the  calcite  areas  are  found  to  be  surrounded  by  a  ground- 
mass  with  feldspar  laths  and  the  general  characters  of  that  of  the 
rock  already  described.  The  calcite  amygdules  are  well  defined, 
for  the  most  part,  but  in  some  of  the  greenstones  which  show 
evidences  of  the  most  dynamic  action,  there  are  areas  in  which  the 
calcite  has  replaced  parts  of  the  rock,  thus  enclosing  feldspar 
laths. 

TUFFS.  The  best  development  of  the  tuffaceous  type  is  at  the 
head  of  Southern  Harbor,  although  the  tuffs  also  occur  on  Crab- 
tree  Point  and  the  southwestern  shore  of  the  island.  In  the  most 
aphanitic  rock  of  this  type,  the  diabasic  structure  is  wholly  want- 
ing, and  the  rock  is  composed  of  an  aggregate  of  secondary 
minerals  similar  to  those  found  in  the  other  types.  These  charact- 
ers may  express  a  complete  alteration  of  the  diabasic  rock,  but 
more  probably  indicate  an  original  difference.  Other  specimens 
are  plainly  tuffaceous,  even  megascopically,  and  the  coarse  breccia 
occurring  just  south  of  where  the  second  stream  enters  South- 
ern Harbor  from  the  east,  is  doubtless  a  tuff  breccia. 

The  greenstone  complex  thus  resolves  itself  into  a  series  of  di- 
abase flows  with  accompanying  pyroclastics.  In  places,  these  di- 
abases may  have  been  porphyritic,  while  in  the  southern  part  of 
the  area,  they  were  distinctly  characterized  by  gas  cavities,'  now 
filled  with  amygdules  of  calcite.  The  general  strike  of  the  series, 
as  inferred  from  the  succession  of  the  different  types,  is  somewhat 
north  of  east. 

The  diabasic  or  ophitic  structure  has  been  thought  to  often  ex- 
press crystallization  under  surface  conditions.  Lowinson-Less- 


16  GEOLOGY   OF   THE    FOX    ISLANDS. 

ing*1  considers  diabases  to  have  been  submarine  effusives  at  consid- 
erable depth,  and  augite-porphyrites  the  terrestrial  flows  from  a 
magma  of  similar  composition.  Such  a  relation  is  thought  bj- 
Barrois2  to  have  held  in  the  Silurian  volcanic  series  of  Brittany. 
This  view  interprets  the  diabasic  structure  as  expressive  of  con- 
ditions of  crystallization  intermediate  between  those  obtaining  in 
intrusive  masses  and  those  of  sub-aerial  effusives.  Nevertheless, 
the  tuffaceous  and  amygdaloidal  phases  of  the  North  Haven  di- 
abases show  that  the  conditions  were  essentially  superficial  and 
truly  volcanic.  The  structure  of  these  rocks  as. described  in  the 
following  pages  also  adds  to  the  weight  of  evidence  as  to  the  vol- 
canic character  of  this  greenstone  complex. 

COLUMNAR  STRUCTURE.  Throughout  the  greater  part  of  the 
area,  the  aphanitic  and  amygdaloidal  diabases  are  characterized 
by  a  columnar  structure.  Seen  in  cross  section  only,  as  on  a 
glaciated  surface  or  low  cliff  on  the  shore,  the  rock  appears  to  be 
divided  into  irregular  ellipsoidal  masses  or  lenses,  from  a  few 
inches  to  two  feet  in  major  diameter.  These  masses  of  compact 
rock  are  embedded  in  a  matrix,  which  is  a  schistose  phase  of  the 
same  rock,  and  the  less  resistance  of  the  matrix  to  weathering 
agencies  gives  prominence  to  the  structure,  the  oval  sections  often 
being  surrounded  by  narrow  crevices.  There  is  considerable 
variation  in  the  perfection  of  the  structure.  On  especially  favor- 
able exposures,  the  best  one  being  a  vertical  cliff  caused  by  a  local 
fault,  the  weathering-  has  proceeded  to  such  an  extent  that  the 
masses  of  more  compact  greenstone  project,  and  remind  one  of 
stumps  of  small  trees  in  an  inclined  position.  In  short,  the  structure 
is  seen  to  be  that  of  irregular  cylinders  of  indeterminate  length. 

Somewhat  similar  structures  have  been  observed  in  the  green- 
stone schists  of  Michigan3,  of  Canada4,  and  of  Saxony5,  and  in  the 
diabase  amygdaloids  of  Vogtland6.  In  all  of  the  occurrences,  how- 


1.  Bull.  Soc,  Beige,  de.  Geol.  Vol.  2,  1888,  p.  86. 

2.  Bull.  geol.  carte  France.  No.  7,  1889,  p.  61. 

3.  Williams:     Bull.  U   S.  Geol.  Survey,  62,  p.  166. 

4.  Lawson:     Report  Geol.  and  Nat.  Hist.  Survey  of  Canada,  1885,  C  C.  p.  52. 

5.  Rothpletz  :     Zeitschr.  Deutsch,  geol.  Gesell.,  vol.  31,  pp    374-397.  1879. 

6.  Dathe:     Jahrbuch  K.  Preuss.  geol.  Landesanstalt,  1883,  p.  432. 


NORTH  HAVEN  GREENSTONE  COMPLEX.  17 

ever,  the  structure  is  usually  ellipsoidal  rather  than  cylindrical; 
and  where  irregular  masses  are  present  they  interlock  as  in  a 
mosaic,  which  is  not  characteristic  to  any  extent  of  the  North 
Haven  diabases. 

The  origin  of  the  structure  in  the  case  of  the  Vogtland  amygda- 
loid is  believed  by  Dathe  to  be  contraction  during  the  consolida- 
tion of  the  diabase  flow  and  in  support  of  this  view,  a  concentric 
arrangement  of  the  anrygdules  parallel  with  the  surface  of  the  ellip- 
soids is  cited.  No  such  arrangement  was  observed  in  the  diabases 
of  North  Haven,  which  otherwise  have  many  points  of  similarity 
with  the  Vogtland  rock.  Such  an  origin,  moreover,  would  fail  to 
account  for  the  schistose  matrix  between  the  columns. 

A  dynamic  origin,  that  of  brecciation  in  situ,  has  been  consider- 
ed a  more  natural  explanation  for  the  other  occurrences  mentioned 
above.  In  the  present  case,  however,  the  origin  seems  to  have 
been  compound,  a  true  contraction  parting,  modified  by  dynamic 
action.  At  one  point  on  the  north  shore  of  Southern  Harbor,  a 
few  typical  pentagonal  prisms  are  to  be  seen  in  the  low  cliff,  and 
on  either  side,  the  transition  into  the  phase  already  described  is 
so  gradual  that  no  sharp  line  of  limitation  for  the  prismatic  part- 
ing could  be  drawn. 

Columnar  parting  of  this  type  is  very  common  in  effusive 
sheets,  and  might  be  expected  in  large  masses  of  amygdaloidal 
diabase.  The  more  or  less  vertical  cracks  which  divide  the  mass 
into  prisms  are  due  to  contraction  after  consolidation,  under  the 
influence  of  surface  conditions.  The  cooling  plane  is  the  surface, 
approximately  horizontal,  and  the  hexagonal  prism  is  the  normal 
form,  resulting  from  the  contraction  in  two  directions  chiefly1. 
Usually,  however,  the  number  of  sides  of  the  prisms  varies  from 
three  to  nine.  Such  parting  is  functional  upon  both  quantity  of 
contraction  and  rate  of  cooling  subsequent  to  consolidation;  thus, 
it  is  most  prominent  in  comparatively  crystalline  basic  lavas,  and 
near  the  surface  of  the  lava  mass,  preferably  when  rough  and 
slaggy.  All  of  these  conditions  are  seen  to  have  existed  in  these 
amygdaloidal  diabases.  In  such  a  surface  flow  considerable 


i.  Iddings:  Am.  Jour.  Sci.  Vol.  31,  1886,  p.  321.  Other  literature  on  the  subject 
is,  Bonney:  Quart  Journ.  Geol.  Soc  ,  1876,  p.  140,  and  Reyer:  Theoretiscbe  Geologic,  p. 
90. 


18  GEOLOGY   OF  THE   FOX   ISLANDS. 

irregularity  in  the  form  and  arrangement  of  the  prisms  is  to  be  ex- 
pected. 

Prismatic  parting-  is  believed  by  Walther1  to  be  due  to  cooling 
of  a  lava  stream. in  contact  with  water,  and  thus  to  mark  shore 
conditions.  Such  a  view  might  be  in  harmony  with  the  hypothesis 
of  a  submarine  origin  for  diabase,  already  cited;  but  the  hypoth- 
esis seems  hardly  necessary. 

If  a  lava  mass,  parted  into  these  prismatic  blocks,  be  subjected 
subsequently  to  even  a  slight  amount  of  differential  movement  the 
prisms  w©uld  be  more  or  less  modified  in  form.  The  prismatic 
angles  would  be  rounded,  and  the  prisms  thus  become  rude  cylin- 
ders, in  a  schistose  matrix.  Such  action  might  be  favored  by  an 
ellipsoidal  parting  subsequent  to  the  prismatic,  with  the  long  axis 
of  the  ellipsoid  parallel  to  that  of  the  prism.  This  position  of  the 
ellipsoid  would  follow  from  the  fact  that  in  the  vertical  direction, 
the  resistance  to  contraction  would  be  counteracted  by  the  weight 
of  the  superincumbent  mass2. 

This  hypothesis  of  subsequent  dynamic  action  as  applied  to  the 
North  Haven  occurrence  is  supported  not  only  by  the  field  rela- 
tions, but  by  the  microscopical  features  as  well.  Both  the  com- 
pact columns  and  the  interstitial  schistose  portions  are  seen  to 
have  suffered  dynamic  action,  but  in  different  degrees.  In  the 
former,  the  calcite  filling  the  old  gas  cavities  shows  a  slight 
amount  of  deformation  marked  by  curved  cleavage  cracks,  while 
the  feldspar  laths  are  always  bent.  In  the  schistose  matrix,  the 
calcite  amygdules  have  been  shattered,  a  mosaic  replacing  the 
large  calcite  units,  and  the  feldspars  are  broken.  The  amygdules 
are  also  less  sharply  defined  and  the  amount  of  calcite  is  relatively 
greater  in  the  schistose  phase.  Such  dynamic  action,  then,  was 
not  only  subsequent  to  the  formation  of  the  feldspar  laths  and  the 
consolidation  of  the  rock,  but  later  than  the  filling  of  the  amygda- 
loidal  cavities  of  the  porous  lava. 

In  many  cases,  the  structure  of  the  diabase  may  be  ellipsoidal, 
rather  than  columnar,  but  the  latter  is  the  more  characteristic  for 
the  area.  The  former  could  result  from  a  similar  dynamic  modifi- 


1.  Walther:     Jahrbuch.  K.  K,  geol.  Reichanstalt  vol.  36,  1886,  p.  295. 

2.  Iddings:     op.  cit. 


NORTH  HAVEN  GREENSTONE  COMPLEX.  19 

cation  of  a  mass  of  lava  parted  into  shorter  prisms  or  even  ellip- 
soids. At  one  locality  these  structures  can  be  compared  with  the 
concentric  weathering*  in  a  dike,  and  the  contrast  is  such  as  to 
allow  no  confusion  of  the  three  different  structures. 

AGE.  The  age  of  these  diabasic  lavas  is  determined  as  pre- 
Niagara  from  their  relations  to  the  conglomerate  at  the  base  of 
the  Niagara  sediments.  The  contact  is  well  exposed  on  the  shore 
of  Southern  Harbor,  where  the  amygdaloidal  diabase  underlies  the 
conglomerate  which  dips  to  the  south  at  an  angle  of  about  60°. 
The  extremely  disintegrated  character  of  the  diabase  at  this  point 
and  the  apparent  enclosing  of  large  lenses  of  this  rock  by 
clastic  material  may  indicate  superficial  disintegration  before  the 
deposition  of  the  Niagara.  In  fact  such  action  would  be  expected 
in  the  case  of  these  vesicular  lavas. 


NIAGARA  SEDIMENTS. 

DISTRIBUTION.  The  area  occupied  by  these  sedimentary  rocks 
is  smaller  than  that  of  any  other  series  on  the  Fox  Islands.  The 
principal  exposure  of  the  Niag-ara  series  is  a  wedg-e-like  area  ex- 
tending- east  from  the  shore  of  Southern  Harbor,  where  it  is  less 
than  a  quarter-mile  in  width,  across  the  Ames  Knob  peninsula, 
with  the  apex  of  the  wedge  east  of  the  mud  flat  and  near  the 
road  from  North  Haven  villag-e  to  Pulpit  Harbor.  Its  full  extent 
is  thus  less  than  a  mile  in  leng-th,  while  the  greatest  width  is  ex- 
posed on  the  shore  of  Southern  Harbor.  These  fossiliferous  sedi- 
ments are  also  found  in  a  narrow  strip  on  the  western  end  of 
Stimpson's  Island.  Throug-hout  both  areas,  the  surface  is  low  and 
the  rocks  are  mostly  concealed,  except  on  the  shores. 

SOUTHERN  HARBOR  SECTION.  The  best  section  of  the  Niag-- 
ara sediments  is  found  on  the  shore  of  Southern  Harbor,  just 
northwest  of  Ames  Knob.  Here  the  basal  member  is  in  immedi- 
ate contact  with  the  greenstone  to  the  north.  The  upper 
members  are  about  seventy  feet  from  the  overlying-  volcanics,  but 
this  break  is  in  part  represented  a  short  distance  further  east  by 
red  shales  with  interbedded  conglomerate.  The  section  was 
measured  at  low  tide,  and  is  much  more  complete  than  that  exposed 
above  the  hig-h  water  mark.  The  section,  in  descending-  order,  is 
as  follows: 

Sandy  shale  and  quartzite,  65 

Concealed,  70 

Gray  shale,  49 

Conglomerate  and  shale,  12 

Red  shale,  55 

Conglomerate,  1 

Red  shale,  10 

Gray  shale,  37 

Concealed,  17 

•20 


NIAGARA   SEDIMENTS.  21 

Gray  shale  and  limestone,  71 

Quartz-porphyry,  intrusive  sheet,  10 

Concealed,  16 
Gray  limestone,  weathering-  brown,  and  shale,     104 

Grit,  coral  fragments,  1/i 

Sandy  limestone,  4 

Concealed,  42 

Basal  conglomerate,  16 

580X  feet. 

The  general  strike  of  the  members  of  the  series  varies  from  N. 
85°  E1.  near  the  base  to  ^".  40°  E.  in  the  upper  members  in  the 
shore  section.  Higher  beds,  exposed  on  the  north  slope  of  Ames 
Knob,  however,  give  a  strike  similar  to  that  of  the  basal  mem- 
bers. The  dip  is  from  50°  to  60°  to  the  south  and  even  steeper  at  a 
few  points.  To  the  east,  on  the  west  shore  of  the  mud  flat,  the 
lower  beds  of  the  series  have  much  the  same  sequence  and  strike, 
but  the  upper  portions  show  some  slight  folding  and  displacement, 
so  that  the  strike  and  dips  vary  somewhat  from  those  of  the  more 
regular  section,  already  given,  where  these  minor  folds  are  less 
apparent. 

FAUNA.  The  paleontology  of  this  series  of  sediments  has  been 
carefully  studied  by  Prof.  C.  E.  Beecher  of  Yale,  and  the  follow- 
ing full  quotation  is  made  from  his  article  already  referred  to2. 

"The  paleontological  evidence  furnished  by  the  specimens  from 
North  Haven  apparently  does  not  admit  of  a  distinct  sub-division 
of  the  rock  series  into  recognized  periods.  A  large  proportion  of 
the  species  clearly  points  to  a  correlation  with  the  Niagara  of  New 
York,  and  to  this  period  they  are  here  referred.  The  presence  of 
quite  a  number  of  characteristic  Clinton  species  is  a  disturbing 
element  in  any  attempt  to  draw  exact  parallelisms  with  the  New 
York  sub-divisions  of  the  Upper  Silurian.  From  present  data, 
however,  it  seems  justifiable  to  consider  the  fossiliferous  rocks  at 
North  Haven  as  representing  a  faunal  equivalent  to  the  Clinton 
and  Niagara,  with  a  decidedly  strong  Niagara  facies.  Therefore, 

1.  Referred  to  the  true  meridian,  as  the  magnetic  variation  is  over  i6c  W. 

2.  Am.  J.  Sc.,  vol.  43,  pp.  416-418.   1892. 


22  GEOLOGY   OF  THE   FOX   ISLANDS. 

the  broader  term  Niagara  will  more  correctly  express  the  chrono- 
logical relations  of  these  strata. 

"The  Clinton  species  are  mostly  confined  to  the  lower  half  of 
the  series.  This  is  of  considerable  significance,  but,  at  the  same 
time,  nearly  all  the  typical  Niagara  forms  are  associated  with 
them  in  abundance.  Another  fact  to  be  noted  is  the  conspicuous 
absence  of  some  species  from  the  entire  series:  as,  Caryocrinus 
ornatus,  Spir if er  niagarensis,  Rhynchotreta  cuneata,  and  Pentamerus 
oblongus.  Similar  discrepances  occur  in  the  Niagara  fauna  at 
Waldron,  Indiana,  and  elsewhere,  and  merely  serve  to  give  dis- 
tinctive features  to  such  faunas." 

On  account  of  subsequent  collection,  the  list  of  fossils  has  been 
revised  by  Prof.  Beecher,  and  through  his  kindness,  the  more  com- 
plete fauna  is  here  given. 

Monograptus  clintonensis  Hall. 
Coenostroma,  2  species. 
Streptelasma  calyculum  HalL 
Cyathophyllum  sp. 
Chonophyllum  niagarense  HalL 
Favosites  venustus  HalL 
Favosites  niagarensis  HalL 
Favosites  f  avosus  HalL 
Cladopora  sp. 
Coenites  ramulosus  HalL 
Syringopora  sp. 
Monticulipora  sp. 
Halysites  catenulatus  Linne. 
Heliolites  spiniporus  HalL 
Crinoid  fragments. 
Ichthyocrinus  laevis. 
Kucalyptocrinus  caelatus. 
Cornulites  sp. 
Tentaculites  sp. 
Encrinurus  punctatus   Wahl. 
Calymene  niagarensis  HalL 
Homalonotus  delphinacephalus  Gr. 
Dalmanites  limulurus  Green. 


NIAGARA   SEDIMENTS.  23 

Ceraurus  niag-arensis  Hall. 

Illaenus  ioxus  Hall. 

Proetus  Stokesi  Hall. 

Proetus  sp. 

Beyrichia,  2  species. 

Leperditia  sp. 

Fenestella  sp. 

Stictopora  sp. 

Ling-ula  lamellosa  Hall. 

Orbiculoidea  sp. 

Orthis  eleg-antula  Dal. 

Orthis  hybrida  Soiv. 

Orthis,  2  species. 

Plectambonites  transversalis  Wahl. 

Plectambonites  (cf.)  sericea  Sow. 

Leptaena  rhomboidalis  Wile. 

Stropheodonta  profunda  Hall. 

Orthothetes  subplanum  Con. 

Chonetes  cornutus  Hall. 

Pentamerus  occidentalis  Hall. 

Whitfieldella  nitida  Hall. 

Meristina  sp. 

Nucleospira  pisum  Hall. 

Atrypina  disparilis  Hall. 

Spirifer  crispus  His. 

Spirifer  sulcatus  His. 

Spirifer  radiatus  Soiv. 

Cyrtina  pyramidalis  Hall. 

Atrypa  reticularis  Linne. 

Atrypa  nodostriata  Hall. 

Rhynchonella  neg-lecta  Hall. 

Rhynchonella  obtusiplicata  Hall. 

Rhynchonella  sp. 

Rhynchonella  (Wilsonia)  sp. 

Nucula  sp. 

Tellinomya  sp. 

Avicula  demissa  Con. 

?  Avicula  subplana  Hall. 


24 


GEOLOGY   OF   THE    FOX    ISLANDS. 


Hydrozoa, 

Actinozoa, 

Crinoidea, 

Annelida, 

Trilobita, 

Ostracoda, 


Avicula  sp. 

Cypricardinia  sp. 

Platyostoma  niagarense  Hall. 

Ivoxonema  sp. 

Pleurotomaria  sp. 

Bellerophon  sp. 

Cyrtolites  sp. 

Murchisonia  sp. 

Hyolithes  sp. 

Orthoceras  annulatum  Sow. 

Orthoceras  subcancellatum  Hall. 

Orthoceras  virgulatum  Hall. 

Orthoceras  (annulated),  2  species. 

Oncoceras  sp. 

Gomphoceras  sp. 

Cytoceras  subcancellatum  Hall. 

SUMMARY. 
3  species.     Bryozoa, 


12 

2 

2 

8 
3 


2  species. 
27 

6       " 
6 


Brachiopoda, 
Pelecypoda, 

*  Gastropoda, 

*  Pteropoda,  1        " 
Cephalopoda,                   8       " 

Total  80  species. 

Prof.  Beecher  states  that  nearly  all  the  corals  are  confined  to 
the  lower  beds,  being-  found  in  a  conglomerate  of  coral  fragments 
together  with  quartz  and  hydromica  schist  pebbles,  and  in  a  lime- 
stone. The  greater  number  of  species  was  found  in  a  shale  near 
the  middle  of  the  series,  on  the  shore  northeast  of  Ames  Knob. 
He  concludes:  "The  fauna  is  evidently  a  rich  one,  and  several  of 
the  unidentified  species  are  probably  new  to  science.  Many  of 
them  show  distinctive  characters  but  the  majority  are  too  frag- 
mentary and  poorly  preserved  for  accurate  description  or  determi- 
nation." 

CONDITIONS  OF  DEPOSITION.  A  series  of  old  sediments  is  of 
the  greatest  value  to  the  geologist,  since  it  affords  an  expression 
of  past  conditions,  both  terrestrial  and  submarine.  Land  topogra- 


NIAGARA    SEDIMENTS.  25 

phy  and  configuration  of  the  sea-floor  have  conditioned  the 
processes  of  sedimentation,  but  the  determination  of  these  con- 
ditions, through  study  of  a  single  section  of  sedimentary  rocks,  is 
not  a  simple  task.  In  the  series  of  about  six  hundred  feet  of 
Niagara  sediments,  as  exposed  on  North  Haven,  the  general 
sequence  is  that  of  basal  conglomeratic  and  arenaceous  beds,  fol- 
lowed by  limestones,  which  grade  upward  into  shales  and  coarser 
sediments. 

The  basal  member  of  the  series  is  a  conglomerate  which  includes 
in  its  lower  portion  larger  well-rounded  masses  of  the  subjacent 
greenstone,  but  in  its  upper  part,  well-rounded  pebbles  of  black  and 
green  slates,  quartzite,  and  vein-quartz,  materials  unlike  the  under- 
lying rock.  Such  a  basal  member  may  be  considered  to  have  been 
deposited  in  a  sea  transgressing  over  a  land  surface  of  rock  more 
or  less  disintegrated,  thus  accounting  for  the  residual  boulders  of 
greenstone.  In  the  later  stages  of  the  deposition  of  the  con- 
glomerate at  this  point,  the  processes  of  sorting  became  more 
efficient  and  the  material  was  derived  from  a  greater  distance. 

The  limestone-shale  series  shows  changing  conditions,  which, 
however,  were  most  constant  during  the  deposition  of  the  hundred 
feet  of  limestone  with  slight  amounts  of  shale,  overlying  the  grit. 
This  latter  bed,  from  its  evenness  of  grain  affords  evidence  of 
perfect  sorting,  and  the  presence  of  coral  fragments  may  be  con- 
sidered to  indicate  breaker  action.  These  calcareous  deposits  are 
quite  fossilferous  and  the  conditions,  therefore,  were  such  as  were 
favorable  to  the  lime  secreting  organisms.  Even  within  the  shaly 
bands,  nodules  of  limestone  occur  and  some  of  the  masses  are 
plainly  coralline. 

Increasing  amounts  of  silt  being  contributed  to  the  sea  terminat- 
ed the  deposition  of  limestone,  and  the  upper  portions  of  the  section 
are  essentially  argillaceous.  In  these  shales  the  remains  of  the 
Niagara  organisms  are  even  better  preserved  than  in  the  lime- 
stone. For  the  most  part,  the  increased  supply  of  sediments 
probably  originated  in  greater  efficiency  of  stream  transportation, 
which,  in  turn,  was  brought  about  by  uplift  of  the  drainage  area, 
tributary  to  this  portion  of  the  Niagara  sea.  That  there  were 
changes  in  wave  and  current  action  is  shown  by  the  presence  of 
thin  conglomerates,  resting  upon  well-washed  surfaces  of  the 


26  GEOLOGY   OF   THK    FOX   ISLANDS. 

shale,  but  grading-  upward  into  the  shale.  At  one  locality,  traces 
of  ripple-marks  can  be  detected  in  the  quartzite  associated  with 
the  shales. 

The  red  shales  which  occur  as  two  important  members  in  the 
shore  section  and  also  higher  up  in  the  series  exposed  a  short  dis- 
tance to  the  east,  deserve  special  mention.  They  are  of  interest 
both  from  their  relations  in  the  section,  being1  at  one  point  in 
direct  contact  with  the  superjacent  volcanics,  and  also  from  their 
close  resemblance  to  certain  fine-grained  tuifs  within  the  volcanic 
series.  This  similarity  is  primarily  one  of  color,  for  none  of  the 
tuffs  quite  approach  the  fineness  of  grain  that  characterizes  these 
shales. 

Two  hypotheses  may  be  presented  to  account  for  the  red  color  so 
noticeable  in  these  shales.  One  is  that  proposed  by  Russell1  as  a 
general  explanation  of  the  red  color  of  certain  formations.  The 
material  from  which  these  shales  are  derived  has  resulted  from 
subaerial  decay  of  crystalline  rocks,  and  in  the  process  of  deposi- 
tion has  not  been  subjected  to  attrition  sufficient  to  separate  the 
ferric  oxide  from  the  silt  or  fine  sand  grains.  Such  an  origin  in- 
volves definite  land  conditions  and  special  shore  conditions.  Of 
these  the  latter  would  probably  vary  most  in  the  deposition  of  a 
series  of  sediments  like  the  one  under  consideration;  and  the  red 
shales  would  indicate  the  absence  of  prolonged  wave  action  to 
which  the  associated  gray  shales  and  clean  quartzites  must  have 
been  subjected.  The  abrupt  change  to  the  well-rounded  conglomer- 
ate, which  is  found  interbedded  with  the  red  shales  in  both  occur- 
rences, is  difficult  to  explain  by  this  hypothesis. 

The  origin  suggested  by  the  field  relations  is  that  the  red  color 
is  due  to  fine  ash  from  neighboring  volcanic  explosions.  Such 
volcanic  dust  could  easily  supply  the  iron  to  color  these  sediments, 
and  this  hypothesis  is  supported  by  the  resemblance  to  the  true 
tuffs  in  the  overlying  volcanic  series.  Prof.  Emerson2,  as  quoted 
by  Russell,  gives  a  similar  explanation  for  the  red  color  of  some  of 
the  sandstones  of  the  Connecticut  Valley. 

Thin  sections  of  this  shale  examined  microscopically  show  the 


1.  Bull.  52,  U.  S.  Geol.  Survey,  p.  44. 

2.  Gazeteer  of  Hampshire  County,  Mass.  1888,  p.  18. 


NIAGARA   SEDIMENTS.  27 

rock  to  be  composed  of  angular  fragments  of  colorless  minerals, 
.01  to  .05  mm.  in  diameter,  enclosed  in  a  matrix  nearly  opaque 
even  in  the  thin  section.  The  mineral  fragments  seem  to  be 
quartz  or  feldspar  for  the  most  part,  with  some  shreds  of  musco- 
vite,  and  small  amounts  of  chlorite  and  calcite.  The  red  color  is 
due  to  a  fine  pigment,  some  oxide  of  iron,  which  not  only  coats  the 
fragments  but  constitutes  a  large  part  of  the  rock,  the  dark  red 
material  appearing  to  be  plentiful  in  fragments  as  well  as  forming 
the  cement.  The  thin  sections  were  treated  with  acids  but  with 
no  appreciable  bleaching  effect. 

The  similarity  to  the  finer  parts  of  the  Hypical  tuffs,  occurring 
within  the  volcanic  series  appears  under  the  microscope  as  well  as 
in  the  field.  The  evidence,  therefore,  seems  to  favor  the  latter 
hypothesis,  namely:  that  the  red  color  is  not  due  to  a  cement  of 
residual  clay,  but  to  finely  comminuted  material,  ejected  from  a 
volcano  and  deposited  in  the  shallow  water  close  to  the  old  shore. 
Thus  the  red  shales  would  present  the  earliest  evidences  of  the 
volcanic  activity  of  the  region,  an  activity  which  later  resulted  in 
the  red  and  gray  lavas,  which  overlie  the  Niagara  sediments. 


CALDERWOOD'S  NECK  SCHISTS. 

DESCRIPTION  AND  DISTRIBUTION.  The  rocks  included  in  this 
series  are  dark  quartzitic  slates,  often  rusty  on  the  weathered 
surface,  banded  schists,  quartzose  but  varying  in  color  and  grain, 
and  quite  massive  quartzites.  These  rocks  occupy  the  northern 
half  of  Calderwood's  Neck,  with  the  exception  of  the  narrow  part 
west  of  Carver's  Cove.  Felsitic  rocks  occur  in  close  proximity  to 
the  granite  and  volcanic  rocks  and  undoubtedly  represent  contact 
phases  of  members  of  this  Calderwood's  Neck  series.  These  are 
mottled  or  variegated,  the  general  color  being  a  light  green.  In  the 
thin  section,  they  are  seen  to  be  composed  of  nearly  pure  epidote 
with  some  zoisite,  in  the  lighter  areas,  and  of  green  hornblende  in 
the  darker. 

Connected  with  this  larger  area  is  a  narrow  strip  of  similar 
rocks  along  the  western  shore  of  Seal  Cove.  Here  the  rocks  are 
somewhat  more  massive,  being  both  quartzitic  and  slightly  cal- 
careous. Epidote  is  quite  prominent  along  the  bedding  planes, 
giving  a  banded  appearance  in  many  places. 

Rocks  resembling  these  in  general  appearance  occur  elsewhere 
on  Vinal  Haven,  on  the  shore  of  Leadbetter's  Narrows,  and  on  the 
extreme  eastern  side  of  the  island  near  Coombs'  Hill.  In  the 
latter  occurence  the  schists  are  actinolitic  in  appearance,  except 
where  in  contact  with  a  granite  dike.  Here  there  is  a  marked 
bleaching  of  the  schist  similar  to  that  observed  at  several  localities 
in  the  Calderwood's  Neck  area.  There  are  also  indications  of  small 
masses  of  these  schists  within  the  diabase  area  of  Barley  Hill,  and 
the  volcanic  area  on  the  western  side  of  Vinal  Haven,  but  these 
areas  are  too  small  to  be  mapped.  On  North  Haven,  the  presence 
of  this  series  is  not  indicated  at  any  point. 

RELATIONS.  The  series  in  its  principal  development  distinctly 
shows  sedimentary  characters.  The  bedding  can  be  observed  even 
at  some  distance  and  seems  quite  constant,  and  the  banding  of  the 
schists  agrees  with  it,  the  strike,  for  the  Calderwood's  Neck  area 

28 


CALDERWOOD'S  NECK  SCHISTS.  29 

being"  usually  within  a  few  degrees  of  north-south,  and  the  dip  30° 
to  40°  to  the  west.  On  the  shores  of  Seal  Cove,  however,  the  rocks 
dip  more  to  the  northwest. 

At  no  point  do  these  schists  come  in  contact  with  the  greenstone 
or  Niagara  sediments  already  considered.  East  of  Seal  Cove  and 
north  of  Mill  River,  the  schists  are  interbedded  with  quartz- 
porphyry,  with  the  relations  apparently  those  of  intrusive  sheets 
of  porphyry  in  the  sedimentary  rocks.  Considerable  metamor- 
phism  has  taken  place  in  the  latter  series,  and  epidote  is  present 
along-  the  joint  planes.  On  the  opposite  shore,  south  of  Perry's 
Cove,  diabase  sheets  and  dikes  mask  the  contact  with  the  over- 
lying- volcanics. 

AGE.  The  contact  relations  prove  that  these  schists  are  older 
than  the  granite  and  diabase  intrusions  and  the  volcanic  series. 
In  their  metamorphic  condition  and  in  the  absence  of  all  traces  of 
fossils,  these  old  sediments  are  in  marked  contrast  to  the  Niagara 
shales.  Their  practically  uniform  composition  for  a  distance  of 
nearly  a  mile  across  the  strike,  on  Calderwood's  Neck,  show  the 
condition  of  their  deposition  to  be  wholly  different  from  those  of 
Niagara  time,  since  in  the  Niagara  section  of  six  hundred  feet,  the 
sediments  show  great  variety.  It  seems,  probable,  therefore,  that 
these  schists  may  represent  an  older  sedimentary  series,  and  with 
the  North  Haven  diabase  flows  constitute  the  oldest  part  of  the 
Fox  Islands. 


THOROUGHFARE   VOLCANICS. 

DISTRIBUTION.  Anyone  visiting-  the  village  of  North  Haven, 
which  is  one  of  the  many  Maine  summer  resorts,  or  even  sailing- 
through  the  Fox  Islands  Thoroughfare,  cannot  fail  to  notice  the 
red,  yellow  and  purple  rocks  that  form  the  bold  shores.  These  are 
the  volcanic  rocks,  most  typically  developed  on  North  Haven,  but 
also  extending  to  the  northern  shores  of  the  southern  island.  The 
two  hills  northwest  and  northeast  from  the  village,  mark  the 
northern  limit  of  this  volcanic  series  in  its  chief  development;  and 
throughout,  these  more  resistant  rocks  determine  a  bolder  topogra- 
phy than  that  seen  to  the  north.  On  the  southern  side  of  the 
Thoroughfare,  the  series  continues  to  the  smaller  portion  of  Cal- 
derwood's  Neck,  while  the  extreme  southwestern  limit  of  the  area 
is  at  Brown's  Head.  This  main  area,  then,  is  crescent-shaped, 
concave  southward.  Similar  volcanics  form  the  peninsula  ending 
in  Indian  Point,  as  well  as  the  five  islands  that  lie  to  the  east  and 
south. 

DESCRIPTION  OF  ROCK-TYPES.  The  area  just  defined  has  been 
termed  one  of  volcanic  rocks.  However,  the  determination  of  the 
volcanic  nature  of  these  rocks,  so  different  in  appearance  from  the 
lavas  and  other  products  of  recent  volcanic  action,  depends  upon  a 
careful  investigation  of  their  many  characters.  Therefore,  a 
description  of  their  structure  and  composition  is  necessary,  and 
this,  together  with  a  statement  of  the  field  relations,  will  con- 
tribute to  the  determination  of  the  conditions  of  the  volcanic 
activity. 

The  two  great  classes  of  volcanic  rocks  are  the  lavas  and  the 
pyroclastics,  and  the  usual  gradation  from  the  one  type  to  the 
other  is  found  among  the  Thoroughfare  volcanics.  The  lavas  of 
this  series  include  several  distinct  types  and  deserve  careful  study, 
since  they  furnish  what  evidence  there  is  as  to  the  nature  of  the 
magma  from  which  these  volcanics  originated.  Among  these 
lavas  are  found  old  andesites,  porphyries,  and  amygdaloids. 

30 


THOROUGHFARE   VOLCANICS.  31 

ANDESITKS.  A  rock  very  prominently  exposed  on  Ames  Knob, 
the  bold  hill  northwest  of  North  Haven  village,  is  one  of  a  dark 
gray  color,  often  with  a  slight  purple  tinge.  It  is  porphyritic 
with  well  defined  phenocrysts  of  feldspar,  yellow  to  greenish 
white,  and  at  one  locality  on  the  north  slope  of  the  Knob,  the  rock 
is  quite  beautiful  in  appearance.  The  abundant  feldspar  pheno- 
crj^sts  are  in  striking-  contrast  to  the  almost  black  groundmass  of 
the  rock,  which  much  resembles  ohe  variety  of  the  "porfido 
antico".  Typically,  this  Ames  Knob  rock  is  compact,  but  amygda- 
loidal  and  even  drusy  phases  also  occur. 

In  the  thin  section,  the  phenocrysts  and  ground-mass  appear 
well  differentiated.  The  feldspar  crystals,  with  the  extinction 
angles  of  labradorite  approaching-  andesine,  are  tabular  and  of 
good  size,  with  albite-twinning-  and  zonal  structure.  Although 
the  feldspar  phenocrysts  are  plentiful,  few  are  unaltered.  Aggre- 
gates of  epidote  and  pennine,  with  some  clear  grains  of  secondary 
feldspar,  have  replaced  the  original  feldspar  without  affecting  the 
crystal  outlines.  The  ferro-magnesian  constituents  have  been 
completely  altered  and  phenocrysts  that  appear  well  defined  in 
ordinary  light,  are  seen  in  polarized  light  to  be  composed  of 
chlorite  and  calcite.  The  outlines  suggest  both  hornblende  and 
pyroxene.  These  latter  phenocrysts  are  older  than  certain  of  the 
feldspar  crystals  into  which  they  project. 

The  ground-mass  is  composed  of  feldspar  laths,  with  a  notice- 
able hyalopilitic  or  felted  texture,  as  well  as  a  fluidal  arrangement 
near  some  of  the  phenocrysts.  The  light  brown  base  is  isotropic 
in  places  and  much  darker  where  included  between  several  feld- 
spar crystals.  Magnetite  occurs  throughout  the  ground-mass  in 
small  grains  and  larger  masses  are  associated  with  the  ferro- 
magnesian  phenocrysts. 

In  addition  to  the  altered  crystals  of  feldspar,  there  are  areas  of 
epidote,  roughly  spherulitic,  with  some  zoisite,  and  areas  of 
chlorite,  both  of  which  by  their  outlines  suggest  irregular  amyg- 
dules.  However,  the  laths  of  the  ground-mass  show  no  parallel- 
ism with  the  borders  of  these  areas;  and  in  a  few  cases,  the 
secondary  minerals  surely  represent  an  alteration  which  has  ex- 
tended beyond  the  outlines  of  the  original  crystals. 

This  porphyritic  rock  presents  the   characters  of  an  andesite> 


32  GEOLOGY   OF   THE    FOX   ISLANDS. 

especially  in  the  texture  of  its  ground-mass.  Upon  the  determina- 
tion of  the  darker  phenocnrsts  would  depend  whether  it  is  to  be 
considered  as  originally  a  hornblende-  or  pyroxene-andesite.  Com- 
parison, however,  with  types  to  be  described  later  favors  the  de- 
termination as  a  pyroxene-andesite1. 

The  rock  well  exposed  on  the  southern  part  of  the  hill  just 
north  of  North  Haven  village  is  compact,  very  dark  although  of 
a  purple  tone,  and  quite,  fresh  in  appearance.  The  feldspar 
crystals  are  apparent  only  by  reason  of  their  bright  cleavage  faces, 
and  dark  green  phenocrysts  are  also  noticeable.  In  the  thin  sec- 
tion, this  rock  proves  to  be  of  special  interest  from  the  opportuni- 
ties it  affords  for  the  study  of  the  processes  of  alteration2. 

The  feldspar  phenocrysts  exhibit  the  albite-lamellation,  have 
the  extinction  angles  of  labradorite,  and  are  beautifully  zonal, 
being  slightly  more  basic  toward  the  center.  Many  of  the  crys- 
tals have  an  altered  core,  but  are  perfectly  idiomorphic,  showing 
that  the  clear  outer  zone  is  original.  (Plate  I,  Fig.  3.)  The 
light  green  pyroxene  phenocrysts  have  the  high  extinction  angle 
of  augite,  and  orthopinacoidal  twinning  is  the  rule,  being  often  re- 
peated in  the  same  crystal.  The  augite,  in  part,  is  altered  to  pen- 
nine.  Both  feldspar  and  augite  phenocrysts  have  a  zonal  arrange- 
ment of  inclusions,  showing  changing  rates  of  crystallization  in 
the  later  stages.  The  ground-mass  is  less  andesitic  in  character 
than  that  of  the  rock  described  above  and  is  full  of  minute  mag- 
netite grains.  Some  interstitial  areas  of  glass  appear  within 
groups  of  feldspars,  while  elsewhere,  faint  micropoikilitic  areas 
are  common  in  the  ground-mass. 

Other  andesites  are  found  which  are  of  a  dark  red  color,  both 
on  the  weathered  surface  and  the  fresh  fracture.  The  bright  feld- 
spar phenocrysts  are  also  often  tinged  with  red,  and  although  this 


1.  The  use  of  the  term  andesite  for  these   Paleozoic  lavas  is   not  in  accord  with   the 
petrographical  nomenclature  of  the  German  school.     Geological  facts,  however,  justify  this 
disregard  of  the  age  element  in  rock  classification,  and  the  usage  adopted  by  the  writer  is 
that  of  many  American  as  well  as    English   petrographers.     vid.     Iddings :      I2th   Ann. 
Rep.  U.  S.  G.  S.,  p.  582;  and  Harker :     Petrology  for  Students,  p.   128.     Rosenbusch  also 
favors   this   reform   in   petrographical   terms,      vid.       Microskopische    Physiographic    der 
Massigen  Gesteime,  1895,  PP-  4~7- 

2.  The  results  gained  from  the  study  of  the  alteration   of  all   the  Fox  Islands  rocks  will 
be  discussed  in  a  subsequent  section. 


THOROUGHFARE   VOLCANICS.  33 

rock  lacks  somewhat  of  the  brilliancy,  it  suggests  the  "porfidp 
rossa  antico"  of  Egypt.  One  specimen  has  a  ground-mass  charact- 
erized by  minute,  irregular  particles  of  red  to  brown  opaque 
material.  The  ground-mass  itself  now  consists  of  micropoikilitic 
areas,  but,  doubtless,  was  originally  very  glassy.  The  feldspar 
phenocrysts  are  less  numerous  than  in  the  other  andesites  and  are 
badly  altered,  but  from  the  amount  of  calcite  present,  a  rather 
basic  character  may  be  inferred.  A  few  phenocrysts  older  than 
the  feldspar,  as  seen  in  ordinary  light  have  the  characteristics  of 
olivine.  (Plate  I,  Fig.  1.)  The  association  with  magnetite,  the 
dark  border  of  iron  oxide,  the  irregular  transverse  cracks,  and  the 
badly  corroded  outlines  of  the  prisms  constitute  the  points  of  re- 
semblance. In  polarized  light  it  is  seen  that  none  of  the  olivine 
remains,  but  that  these  phenocrysts  consist  of  fibrous  serpentine, 
with  fibers  normal  to  the  transverse  cracks,  which  are  bordered 
by  the  opaque  oxide.  Small  grains  of  quartz  plainly  secondary 
occur  within  the  serpentine  mass.  Not  only  is  magnetite  included 
within  these  olivine  phenocrysts,  but  also  large  hexagonal  prisms 
of  apatite,  the  oldest  constituent  of  the  rock. 

From  the  former  presence  of  olivine  as  an  original  constituent, 
this  rock  would  naturally  be  considered  the  most  basic  of  the 
North  Haven  volcanics,  and,  therefore,  a  chemical  analysis  has 
been  made  by  Messrs.  K.  W.  Magruder  and  W.  A.  Jones. 

SiO2                                             .      '  *         .  63.25 

Ti  O2            ....         .         .  .      trace 

A12O3 .  14.89 

Fe2O3           .         .         .         .         .      •'..".  .       6.54 

Fe  O         .         .         .         .         .         .         .  none 

MgO            .        IS      ;         .         .        ;  .       0.82 

CaO         „-•••     '..      'v        .         .         .         /  0.59 

K2O             ...     .         .         .         .         «  .       4.78 

Na2O       .         .,      ~.         .         .         .         .  ;         4.47 

P2O5             .         .         .         .'        .       .  .  .       0.61 

H2  O  (by  Pb  O)       .         .         .         .         .  2.67 

C  O2              .         .         .         .         .         ...  0.78 

Loss  by  ignition,  not  accounted  for  by 

H2  O  and  C  O2,   and  including  small 

amounts  of  Cl  &  F  of  apatite          ..        ..  0.53 

99.93 


34  GEOLOGY   OF  THE   FOX   ISLANDS. 

In  its  mineralogical  composition,  this  rock  approaches  the 
basaltic  type,  but  as  the  analysis  shows  is  somewhat  too  acid. 
The  olivine  phenocrysts,  moreover,  are  not  very  numerous  and 
there  is  reason  to  regard  this  as  simply  an  olivine-bearing  phase  of 
the  andesite. 

Another  specimen  of  the  red  andesite,  collected  on  the  same 
slope  of  Ames  Knob,  has  the  large  feldspar  phenocrysts  quite  un- 
altered, and  with  extinction  angles  indicating  basic  andesine. 
The  ferro-magnesian  constituent  is  not  present  except  as  well  de- 
fined prismatic  areas,  now  composed  of  chlorite  and  calcite.  There 
is  much  magnetite  and  some  ilmenite,  and  apatite  is  associated 
with  the  magnetite  as  inclusions  in  the  feldspars,  and  also  as  long 
prisms  in  the  ground-mass.  One  crystal  of  titanite  or  sphene  was 
observed,  altered  at  the  edges  into  calcite  and  an  opaque  oxide. 

The  ground-mass  contains  feldspar  laths  more  acid  than  the 
phenocrysts,  usually  simple  but  sometimes  twinned,  and  often 
irregularly  terminated.  There  are  also  present  laths  and  trichitic 
masses  of  a  brown  color.  The  flow  structure  is  well  developed 
near  the  phenocrysts  (Plate  I,  Fig.  2.)  but  elsewhere  the  parallel- 
ism of  the  elements  of  the  ground-mass  is  less  apparent.  With 
crossed  nicols,  the  micropoikilitic  structure  of  the  ground-mass  can 
be  seen,  but  this  seems  to  be  due  to  secondary  crystallization. 

Another  of  the  dark  gray  andesites  is  of  much  coarser  texture, 
with  larger  phenocrysts  of  yellowish  white  feldspar.  In  the  thin 
section,  these  feldspars  are  seen  to  have  suffered  the  usual  altera- 
tion to  epidote  and  zoisite,  and  the  porphyritic  darker  constituents 
are  also  wholly  altered.  They  have  the  opaque  resorption  borders 
and,  therefore,  probably  represent  either  hornblende  or  mica, 
some  cross-sections  approximating  the  angles  of  hornblende. 
Magnetite  is  very  abundant  and  apatite  occurs  in  some  of  the  more 
basic  areas. 

The  ground-mass  is  lighter  colored,  although  containing  minute 
grains  of  magnetite.  Microlitic  laths  of  feldspar  constitute  a 
thin  felt,  and  the  clear  base  has  a  very  slight  effect  upon  polarized 
light,  even  appearing  isotropic  in  great  measure.  It  is  probable 
that  this  rock  is  essentially  a  hornblende-andesite,  rather  more 
acid  than  many  of  the  other  lavas  here  described. 


THOROUGHFARE   VOI.CANICS.  35 

PORPHYRIES.  A  dark  green  rock  associated  with  these  ande- 
sites  has  many  of  the  characters,  both  megascopic  and  microscopic, 
similar  to  those  described  above,  but  structural  differences  in  the 
ground-mass  seem  to  warrant  a  separation  of  this  type  from  the 
andesitic.  The  feldspar  phenocrysts  are  almost  wholly  altered, 
but  the  twinned  crystals  of  monoclinic  pyroxene  are  quite  fresh, 
some  chlorite  being-  present  as  a  product  of  the  partial  alteration. 
The  pyroxene  crystals  are  nearly  colorless  in  the  thin  section  and 
have  the  optical  orientation  of  augite.  The  ground-mass  is  holo- 
crystalline,  but  very  fine-grained  and  approaches  the  ophitic 
structure  of  diabase.  The  rock,  therefore,  may  be  called  a  di- 
abase-porphyry1. 

At  the  base  of  Ames  Knob  on  the  south  side  occurs  another 
porphyritic  rock  of  a  green  color.  The  yellow  feldspars  are  very 
numerous,  the  porphyritic  character  of  the  rock  becoming-  some- 
what masked  by  their  abundance  and  the  presence  of  black  dots  of 
a  secondary  mineral.  This  rock  is  essentially  holocrystalline  and 
since  the  feldspar  laths  of  the  ground-mass  show  some  fluidal 
arrang-ement  around  the  phenocrysts,  this  type  may  be  termed  an 
andesite-porphyry.  The  feldspar  phenocrysts  are  labradorite,  and 
augite  is  represented  sparingly,  mostly  altered  to  chlorite,  but 
giving-  the  hig-h  extinction  ang-le  where  unaffected.  Chlorite  also 
forms  the  matrix  of  the  ground-mass.  The  black  spots  seen  in  the 
hand-specimen  are  found  to  be  areas  of  very  finely  divided  zoisite 
of  a  lig-ht  brown  color  in  the  thin  section.  One  of  these  ag-gre- 
g-ates  is  at  the  center  of  an  altered  feldspar  crystal.  Kpidote  and 
secondary  feldspar  are  other  alteration  products  in  this  andesite- 
porphyry. 

Other  rocks  within  this  area  appear  quite  holocrystalline.  The 
darker  silicates'  are  replaced  by  secondary  minerals,  chlorite  and 
calcite,  and  the  feldspars  are  badly  altered.  Much  ilmenite  is 
present,  with  associated  leucoxene,  which,  in  part,  has  the  char- 
acters of  sphene,  and  the  rhombohedral  parting-  of  the  ilmenite  is 
very  finely  exhibited  throug-h  this  alteration.  The  ground-mass  is 


i.  Porphyry  is  used  here  purely  as  a  term  indicative  of  structure.  Its  retention  in  this 
sense  throughout  all  the  rock  families,  thus  abandoning  the  term  porphyrite,  is  the  usage 
favored  by  Cross,  Iddings,  and  Pirsson. 


36  GEOLOGY   OF   THE    FOX    ISLANDS. 

microgranitic  with  no  evidence  of  any  flow  structure,  such  as 
characterizes  most  of  the  other  rocks  described.  This  rock  type 
corresponds  to  the  porphyrite  of  many  petrographers  and  may  be 
termed  either  a  diabasic  or  a  dioritic  porphyry. 

In  one  diabase-porphyry,  with  labradorite  phenocrysts,  there  are 
granuar  patches  of  more  acid  feldspar.  The  interlocking*  grains 
are  much  larger  than  the  laths  of  the  ground-mass  and  contain 
relatively  more  magnetite  and  hematite  than  the  rest  of  the  rock. 
This  striking-  variation  in  composition  and  texture  doubtless 
represents  a  primary  segregation  within  the  crystallizing-  lava. 

Within  this  North  Haven  area,  there  also  occur  quartz- 
porphyries,  which  from  their  association  with  equivalent  pyro- 
clastics  are  to  be  considered  as  belonging-  to  the  volcanic  series. 
The  meg-ascopic  determination  of  these  yellow  porphyries  is  not 
difficult,  since  the  quartz  phenocrysts  are  plainly  seen.  These 
quartzes  are  prominent  in  the  thin  section,  and  furnish  beautiful 
examples  of  well  defined  phenocrysts,  the  sharp  outlines  of  the  bi- 
pyramids  being  interrupted  only  by  the  deep  embayments.  In 
some  instances,  this  effect  of  mag-matic  corrosion  has  been  in  part 
remedied  by  the  subsequent  addition  of  quartz,  which  seems  to  be 
of  the  same  period  of  crystallization  as  the  ground-mass.  The 
feldspar  phenocrysts  have  altered  to  calcite  and  white  mica,  and 
secondary  muscovite  also  clouds  the  interlocking-  grains  of  the 
ground-mass.  In  a  few  cases,  these  grains  are  of  twinned  feld- 
spar, and  in  others,  they  have  small  areas  of  quartz  at  the  center 
surrounded  by  micropeg-matitic  inter  growths.  Similar  intergrowths 
constitute  a  marked  feature  of  another  quartz-porphyry,  the 
volcanic  character  of  which  is  less  certain.  Here,  however,  the 
micropeg-matitic  areas  envelop  the  quartz  phenocrysts  and  are  in 
crystalline  continuity  with  them,  forming-  wide  borders,  which 
often  reach  from  one  crystal  to  another. 

AMYGDALOIDS.  Meg-ascopically,  one  of  the  most  noticeable 
rocks  on  North  Haven  is  an  amygdaloid  which  occurs  on  the  shore 
of  the  Thoroughfare.  The  ground-mass  of  the  rock  is  gray,  and 
the  elongated  amygdules  are  very  plentiful,  being  composed  of 
white  and  pink  quartz.  On  weathered  surfaces,  the  rock  often  ap- 
pears very  vesicular. 


THOROUGHFARE   VOLCANICS.  37 

The  ground-mass  when  studied  microscopically  resolves  itself 
into  short  feldspar  laths,  badly  altered,  in  what  may  have  been 
originally  a  glassy  base,  quite  dark  in  color.  The  feldspar  laths 
show  some  parallelism  to  the  edges  of  the  amygdules,  and  also  a 
certain  amount  of  flow-structure.  The  few  phenocrysts  of  feldspar 
have  characteristic  outlines  but  are  now  wholly  altered  to  chlorite,, 
calcite,  and  quartz-feldspar  aggregates. 

The  amygdules  constitute  one-fourth  of  the  rock  section.  The 
structure  is  concentric,  quartz  grains  and  chlorite  forming  the 
outer  zone,  from  which  quartz  grains  or  perfect  hexagonal  prisms 
project  inward.  The  central  portion  is  filled  with  calcite  or  quartz, 
the  latter,  in  a  few  cases,  being  in  roughly  spherulitic  plumes. 
This  rock  may  be  termed  an  andesitic  amygdaloid. 

A  dense  fine-grained  red  rock  occurs  with  the  gray  andesites  on 
Ames  Knob,  and  differs  from  them  in  the  small  number  of  pheno- 
chrysts.  Examined  microscopically,  it  is  found  to  represent  an  ex- 
tremely vesicular  lava,  the  irregular  cavities  having  been  filled 
with  secondary  minerals.  The  base  is  of  a  red  color  but  wholly 
opaque  and  thus  unlike  that  of  the  other  lavas  studied.  One  feld- 
spar crystal  was  observed,  but  the  degree  of  alteration  prevented 
any  determination  of  its  composition. 

The  lavas  which  have  been  described  above  are  the  andesites, 
pyroxene-,  hornblende-,  and  basaltic,  the  andesite-  and  diabase- 
porph}^ries,  and  the  amygdaloids,  comprising  a  series  varying 
quite  widely  in  structure  and  mineralogical  composition  but  pro- 
bably much  less  in  chemical  composition.  Associated  with  these 
lavas  of  intermediate  composition  is  the  quartz-porphyry,  a  much 
more  acid  type. 

PYROCXASTICS.  Associated  with  the  lavas  are  the  volcanic 
fragmental  rocks,  the  pyroclastics.  They  are  igneous  in  origin, 
but  clastic  in  structure  and  thus  connect  the  compact  lavas  with 
the  ordinary  sediments.  So  close,  indeed,  is  the  relationship  and 
the  resemblance  between  these  pyroclastics  that  it  often  is  difficult 
to  determine  to  which  class  some  of  the  members  of  the  North 
Haven  series  belong.  On  the  other  hand,  it  becomes  a  problem 
where  to  draw  the  line  of  separation  between  the  pyroclastics  and 
the  more  normal  sediments,  to  which,  moreover,  the  volcanic  de- 


38  GEOLOGY   OF   THE    FOX    ISLANDS. 

tritus  has  contributed  so  much.  It  is  necessary  to  realize  that  the 
gradation  throughout  is  complete  and  sharp  distinctions  are,  to  a 
great  degree,  artificial. 

The  most  important  of  the  pyroclastics  represented  on  North 
Haven  is  the  volcanic  conglomerate  on  the  slopes  of  Ames  Knob. 
The  more  or  less  rounded  fragments  of  the  different  andesites 
vary  in  size  from  that  of  a  boulder  several  feet  in  diameter  to  that 
of  coarse  sand.  The  cementing  material  is  small  in  amount  and 
its  character  difficult  of  determination;  but,  when  studied  micro- 
scopically, this  matrix  is  seen  to  be  clastic,  consisting  of  angular 
crystal  fragments  and  particles  of  opaque  material  similar  to  that 
forming  the  ground-mass  of  certain  of  the  lavas.  Slight  traces  of 
flowage  may  indicate  that  the  cementing  material  was  of  the 
character  of  a  volcanic  mud,  when  it  filled  the  spaces  in  this  mass 
of  coarser  ejectamenta.  The  large  fragments  show  the  andesite 
structure  in  a  marked  degree,  and  some  closely  resemble  the  dark 
vesicular  lava  last  described. 

A  volcanic  conglomerate  somewhat  finer-grained  occurs  on  Iron 
Point,  and  the  fine  dark  red  material  which  cements  the  fragments 
of  andesites  again  suggests  an  original  volcanic  mud.  In  both  oc- 
currences, the  volcanic  conglomerate  grades  into  a  breccia,  which 
is  usually  of  the  character  of  a  lava  including  blocks  of  older  lavas. 
Such  a  breccia  would  be  a  primary  or  flow-breccia.  In  other  cases, 
however,  the  breccia  seems  to  differ  from  the  volcanic  conglom- 
erate only  in  the  degree  of  rounding  of  the  fragments  and,  there- 
fore, may  be  termed  a  tuff-breccia. 

The  pyroclastics  of  the  North  Haven  series  afford  examples  of 
the  gradation  in  size  of  grain,  from  the  coarse  conglomerate  to  the 
tuff,  composed  of  fine  volcanic  dust.  The  coarser  tuffs  are  identi- 
fied most  easily,  since  the  angular  andesite  fragments  can  be  seen 
on  the  weathered  surface  of  these  tuffs,  which  are  usually  dark 
colored  and  compact,  although  often  quite  resembling  a  sandy 
shale.  As  in  the  conglomerates,  the  included  fragments  have 
been  derived  from  the  various  andesitic  lavas,  the  more  basic,  the 
vesicular,  and  those  containing  the  altered  hornblende  phenocrysts 
with  resorption  rims.  Mineral  fragments  also  occur,  magnetite 
and  ilmenite  being  most  abundant,  and  the  former  characterized 
by  apatite  inclusions.  These  mineral  grains  are  extremely  angu- 


THOROUGHFARE   VOLCANICS.  39 

lar  and  cannot  be  considered  as  having-  resulted  from  disintegra- 
tion of  the  andesites  through  atmospheric  agencies.  The  cement- 
ing- material  is  larg-ely  calcite. 

A  series  of  tuffs  much  finer  in  grain  are  the  compact  pink  rocks 
on  Ames  Point,  which  on  the  weathered  surface  are  lighter 
colored  and  plainly  tuffaceous.  In  the  thin  section,  they  show 
very  few  larger  fragments  of  minerals  or  of  lavas,  but  consist  of  a 
mass  of  almost  colorless  angular  particles.  These  minute  frag- 
ments  seem  to  represent  glass  sherds,  with  conca've  triangular  out- 
lines for  the  most  part,  often  crescent  or  sickle-shaped.  The 
structure  determined  by  this  material  is  that  described  as  the 
"aschen-structur"  by  Mugge1,  and  considered  by  him  to  be  char- 
acteristic for  deposits  of  the  most  finely  comminuted  g-lass  from  a 
volcanic  explosion.  This  ash-structure  somewhat  resembles  the 
rhyolitic  or  fluidal  structure  but  may  be  distinguished,  as  in  the 
present  case,  by  the  fact  that  the  angular  areas  are  distinct  from 
the  matrix  even  in  polarized  lig-ht.  The  fragments  although 
originally  glass,  are  now  cryptocrystalline,  with  a  somewhat  zonal 
arrangement  which  suggests  that  these  triangular  areas  represent 
cavities  between  rounded  grains  rather  than  the  g-lass  fragments 
themselves.  However,  such  zonal  crystallization  has  been  ob- 
served by  Arnold-Bemrose2  in  the  altered  lapilli  of  Carboniferous 
tuffs  and  Mugge3  shows  that  it  is  to  be  expected  in  the  devitrifi- 
cation process,  since  the  pseudomorphism  would  proceed  exactly 
parallel  with  the  outer  boundary  in  an  amorphous  glass.  Further- 
more, a  study  of  the  arrangement  of  the  triangular,  crescent,  and 
filament-like  areas  readily  shows  that  the  complexity  can  hardly 
be  explained  as  due  to  interstices  between  grains.  One  section  ex- 
hibits an  included  fragment  of  a  lava  with  a  quartz  phenocryst, 
associated  with  this  typical  ash-structure.  (Plate  I,  Fig.  4). 
These  fine  tuffs,  therefore,  are  to  be  considered  as  derived  from  a 
rhyolitic  lava. 

Associated  with  the  quartz-porphyry  are  tuffs,  yellow  to  light 
gray  in  color,  and  very  felsitic  but  including  fragments  of  slate 


1.  Neues-Jahrbuch  f.  Min.  etc.     Beil-B.  8.,  p.  648. 

2.  Quart.  Journ.  Geol.  Soc.  London,  vol.  50.,  p.  60. 

3.  Neues-Jahrbuch  f.  Min.  etc.     1896,  vol.  I.,  p.  79. 


40  GEOLOGY   OF   THE   FOX   ISLANDS. 

and  dark  hornblendic  or  chloritic  material.  Quartz  and  feldspar 
grains  can  be  observed  and  portions  of  the  surface  appear  sericitic. 
The  clastic  character  is  even  more  evident  when  the  rock  is 
examined  microscopically.  Corroded  quartzes  with  embayments, 
angular  pieces  of  clear  feldspar,  both  orthoclase  and  twinned 
plagioclase,  short  prismatic  zircons,  and  shreds  of  muscovite  are 
included,  but  the  fragments  that  constitute  the  greater  part  of  the 
tuff  are  different  in  character.  These  are  very  angular  even 
approaching  the  concave  forms  of  the  smaller  glass  particles  de- 
scribed above,  and  consist  of  quartz  or  feldspar  with  undulatory  or 
roughly  spherulitic  extinction.  A  marked  difference  can  be  ob- 
served also  in  ordinary  light  between  these  fragments  and  the 
others  which  are  plainly  fractured  phenocrysts,  and  secondary 
crystallization  seems  to  be  indicated.  Chlorite  and  calcite  are 
prominent  constituents  of  the  matrix  and  the  latter  mineral  occurs 
also  as  an  alteration  product  of  some  of  the  feldspars.  This  rock, 
then,  may  be  termed  a  quartz-porphyry  tuff  and  the  angular  in- 
clusions of  quartzite  and  slate,  doubtless  also  represent  the  ex- 
plosive action  at  the  volcanic  vent. 

Certain  of  the  purple  tuffs  on  Iron  Point  are  quartzitic  in  appear- 
ance and  from  their  perfect  banding  suggest  sedimentation  where 
wave-action  could  sort  the  material.  The  sub-angular  fragments 
are  wholly  of  volcanic  origin,  as  in  the  other  tuffs,  but  the  rock 
represents  a  phase  more  nearly  allied  to  the  more  common  classes 
of  sediments.  An  overlying  conglomerate  so  closely  resembles 
well-washed  conglomerates  of  common  occurrence  as  not  to  de- 
serve to  be  considered  a  pyroclastic. 

SEQUENCE.  As  data  for  the  determination  of  the  history  of  the 
vulcanism  of  this  area,  observations  on  the  stratigraphical  se- 
quence of  the  different  volcanic  products  are  necessary.  A  uni- 
form succession  comparable  with  that  of  sedimentary  rocks  in  a 
limited  area  is  hardly  to  be  expected  in  a  volcanic  series,  and  on 
comparing  several  geologic  sections  of  the  North  Haven  volcanics, 
differences  in  the  sequence  will  be  noted.  While,  however,  detail- 
ed areal  study  emphasizes  these  local  differences,  a  general  se- 
quence is  observed. 

The  lowest  members  of  the   series,   the  dark  gray   pyroxene- 


THOROUGHFARE   VOLCANICS.  41 

andesite  and  andesite-porphyry,  are  most  prominently  exposed  on 
Ames  Knob,  and  to  the  south  of  this  hill  the  sequence  can  be 
traced  without  difficulty.  Above  the  andesitic  lavas,  which  are 
locally  very  vesicular,  occurs  the  volcanic  conglomerate,  a  well  de- 
fined deposit  with  a  general  strike  of  N.  65°  E.  The  included 
fragments  of  the  dark  gray  pyroxene-andesite  and  porphyry  which 
constitute  the  greater  part  of  the  volcanic  conglomerate  afford 
proof  of  the  relative  age  of  the  members  of  the  series.  To  the 
south  and  west,  this  conglomerate  grades  into  breccias,  which 
show  both  the  tuffaceous  and  flow  characters.  The  red  basaltic 
andesites  overlie  the  conglomerate  and  breccia  and  are  in  turn 
followed  by  the  more  acid  hornblende-andesite.  At  this  point  in 
the  series  the  lavas  become  less  prominent  and  on  the  shores  of  the 
Thoroughfare  the  tuffaceous  rocks  are  exposed,  acid  and  basic  tuffs 
being  interbedded.  The  best-bedded  of  these  tuffs  strike  N.  85° 
W.  and  dip  62°  to  the  south.  A  short  distance  to  the  east,  the 
amygdaloidal  flows,  with  amygdules  giving  a  strike  of  N.  80°  W., 
occur  above  the  hornblende-andesite  and  are  often  interbedded 
with  the  tuffs. 

Just  west  of  North  Haven  village,  occur  the  more  acid  mem- 
bers, the  quartz-porphyry  tuffs  and  small  amounts  of  the  quartz- 
porphyry,  while  on  the  shores  of  the  Thoroughfare,  at  the  village, 
the  yellow  quartz-porphyry  is  prominently  exposed.  This  more 
acid  lava  is  not  as  widely  extended  as  the  andesitic  types. 

In  the  eastern  part  of  the  main  area  of  the  volcanics,  on  the 
shores  of  Waterman's  Cove,  much  the  same  succession  of  red  and 
gray  andesites,  andesite-porphyry,  and  breccia  is  found,  with  some 
quartz-porphyry,  which  here  is  doubtless  instrusive.  On  Iron 
Point,  portions  of  the  series  are  exposed  which  would  overlie  the 
rocks  of  the  Ames  Knob  district.  These  are  well  bedded  tuffs, 
striking  N.  80°  K.  and  dipping  58°-61°  S.  With  these  tuffs  occurs 
the  medium-grained  conglomerate  with  well-rounded  pebbles.  It 
is  to  be  noted  that  throughout  the  principal  area  on  North  Haven, 
the  bedding  of  the  pyroclastics  is  quite  constant. 

Across  the  Thoroughfare  on  the  extreme  northern  portion  of 
Calderwood's  Neck,  the  same  sequence  is  found,  the  breccias  over- 
laid by  tuffs  and  the  conglomerate,  the  series  striking  N.  40°  W. 
and  dipping  82°  S.  At  this  locality  the  section  is  continued  even 


42  GEOLOGY   OF   THE    FOX    ISLANDS.  * 

higher  than  on  Iron  Point,  for  breccias  overlie  these  bedded  rocks, 
and  at  the  entrance  of  Seal  Cove,  well-bedded  tuffs  occur  above 
the  breccias  with  approximately  the  same  strike,  and  a  dip  of 
about  50°  S.  This  change  in  strike  from  that  found  to  the  north- 
west on  Iron  Point  must  be  accounted  for  by  folding,  with  possibly 
some  faulting.  To  the  east  on  the  shore  of  Carver's  Cove  the 
lower  of  the  bedded  tuffs  and  the  conglomerate  are  found,  striking 
N.  5°-25°  E.  and  dipping  about  35°  to  the  west.  Again  folding  and 
faulting  are  necessary  structural  elements  in  the  explanation  of 
the  distribution  of  these  beds.  A  short  distance  to  the  south,  two 
parallel  faults  can  be  seen  in  the  shore  cliff.  The  strike  of  these 
vertical  faults  is  N.  45°  W.  and  the  displacement,  which  is  rela- 
tively downward  on  the  southern  side,  amounts  to  somewhat  over 
fifty  feet  in  one  of  the  faults,  while  the  amount  of  throw  in  the 
other  cannot  be  measured,  since  the  lavas  and  breccias,  which 
overlie  the  bedded  tuffs,  afford  no  datum-plane.  The  actual  ob- 
servation of  displacement  at  this  point  strengthens  the  probability 
of  unexposed  faults  existing  to  the  north. 

On  the  western  side  of  Vinal  Haven  Island,  the  upper  part  of 
the  series  is  again  exposed,  consisting  of  andesitic  lavas,  with 
tuffs  and  breccias.  The  well-bedded  tuffs  in  this  part  of  the  area 
strike  N.  18°-25°  E.  and  dip  62°-67°  to  the  southeast.  At  Brown's 
Head,  the  upper  part  of  the  series  is  in  contact  with  the  Vinal 
Haven  acid  volcanics  to  be  discussed  later. 

The  crescent  shape  thus  expresses  the  structure  of  the  North 
Haven  series  in  its  principal  development  as  well  as  the  areal  dis- 
tribution. 

In  the  area  of  similar  volcanics  to  the  east,  along  the  shores  of 
the  Little  Thoroughfare,  the  most  prominent  rock-type  is  a  flow- 
breccia,  with  which  occur  tuffaceous  rocks  of  varying  character. 
There  is  nothing  in  the  sequence,  however,  to  warrant  more  than 
a  correlation  in  general  character,  with  the  rocks  of  the  principal 
area. 

AGE.  The  age  of  the  North  Haven  volcanics  can  be  deter- 
mined only  by  their  relation  to  the  Niagara  sediments.  In  the 
Ames  Knob  locality  the  volcanic  rocks  overlie  the  upper  members 
of  the  Niagara  series  with  about  the  same  strike  and  dip.  The 


THOROUGHFARE   VOLCANICS.  43 

lack  of  any  marked  unconformity  must  be  considered  as  showing- 
that  the  volcanic  outbreaks  followed  the  deposition  of  Niag-ara 
sediments,  before  the  tilting-  of  those  sediments  to  their  present 
position.  The  red  shales  of  the  Niag-ara  series  quite  probably 
represent  old  muds,  which  received  considerable  amounts  of  vol- 
canic dust,  thus  marking-  the  commencement  of,  the  intermittent 
volcanic  activity. 

On  Stimpson's  Island,  however,  there  is  geological  evidence 
which  bears  more  directly  upon  the  ag-e  problem.  A  strip  of  lime- 
stones and  quartzites  less  than  a  hundred  feet  in  width  occurs 
within  the  area  of  volcanic  rocks.  The  strike  and  dip  of  these 
rocks,  N.  60°  W.  and  30°  to  the  northeast,  and  the  thinness  of  the 
section  prevents  exact  correlation  with  the  Niagara  series  on 
North  Haven,  but  the  rocks  very  closely  resemble  the  latter  sedi- 
ments and  the  only  fossils  found,  traces  of  brachiopod  shells  and 
a  crinoid  stem,  are  also  similar  in  appearance  to  the  more  plentiful 
org-anic  remains  of  the  other  series.  These  sediments,  therefore, 
were  deposited  under  similar  conditions  and,  most  probably,  in  the 
same  period  as  those  which  have  been  proved  to  be  Niag-ara, 
althoug-h  not  necessarily  ever  closely  connected  with  the  other 
area.  Volcanic  rocks,  gray  lavas,  underlie  these  quartzites  and 
limestones,  and  tuffs  and  breccia  overlie  them,  so  that  they  must 
be  reg-arded  as  having-  been  deposited  during-  a  time  of  cessation  in 
volcanic  activity,  a  break  not  recorded  in  this  manner  at  any 
other  locality  on  the  Fox  Islands.  It  seems  justifiable,  therefore, 
to  believe  that  the  volcanic  activity  beg-an  in  Niag-ara  time. 

CONDITIONS  OF  VOLCANIC  ACTIVITY.  .  In  any  attempt  to  picture 
the  nature  of  the  vulcanism  that  characterized  this  area  in  Niag-ara 
time,  the  position  of  the  volcanic  center  should  be  determined.  It 
would  be  of  interest  to  know  the  vent  through  which  the  lavas  of 
this  series  were  poured  forth  and  the  f rag-mental  material  ejected. 
However,  it  is  probable  that  the  area  of  North  Haven  volcanics  de- 
scribed above  is  not  more  than  a  frag-ment  of  the  original  area  and 
that  through  the  processes  of  erosion,  many  times  repeated,  .the 
greater  part  of  the  volcano  has  been  cut  away.  In  the  study  of 
this  limited  area,  no  definite  comparisons  of  the  relative  thicknesses 
of  the  volcanic  series  can  be  made,  such  as  would  point  to  the  posi- 


44  GEOLOGY   OF   THE   FOX   ISLANDS. 

tion  oi  the  volcanic  center.  The  breccias  so  prominent  among-  the 
North  Haven  rocks  are  somewhat  agglomeratic  in  character,  but 
their  distribution  is  too  general  to  indicate  accumulation  within  a 
crater.  Topographically,  Ames  Knob  is  suggestive  of  an  old  vol- 
canic plug,  but  the  relations  of  the  rocks  at  this  locality  do  not 
justify  such  a  view.  Thus,  the  evidence  obtained  in  the  study  of 
these  old  volcanics  does  not  warrant  even  conjectures  as  to  the  site 
of  the  vent. 

The  rock-characters  of  these  volcanics  are  somewhat  indicative 
of  the  conditions  under  which  these  rocks  had  their  origin.  The 
character  of  the  crystallization  in  the  andesites  is  expressive  of 
consolidation  at  the  surface  and  the  flow  structure  shows  that 
these  lavas  consolidated  while  in  motion.  The  amygdaloidal 
character  of  several  of  the  lava-flows  resulted  from  the  expansion 
of  escaping  gases  in  the  molten  mass,  while  the  intercalation  of 
beds  of  tuffs  with  the  amygdaloids  shows  that,  at  times,  these 
gases  became  explosive  and  the  eruption  of  lavas  was  interrupted 
by  the  ejection  of  lapilli  and  volcanic  dust,  which  were  deposited 
upon  the  flanks  of  the  volcano.  The  flow-breccias  which  have 
been  described,  may  be  considered  as  having  their  origin  in  the 
mingling  of  two  lava-streams  of  different  eruptions.  The  cooling 
crust  of  a  lava-flow  becomes  broken  into  angular  blocks  and  these 
may  be  caught  up  in  a  later  stream  of  lava,  which  in  its  lower 
portions  may  be  composed  almost  wholly  of  these  angular  frag- 
ments. In  these  flow-breccias,  the  included  blocks  and  the  matrix 
may  be  quite  different  in  character,  masses  of  the  gray  andesite 
being  usually  embedded  in  the  red  andesite,  which  often  shows 
flowage  around  and  between  these  included  blocks.  In  other 
cases,  the  lava  blocks  have  been  cemented  by  tuff  material  or  by 
volcanic  mud.  The  volcanic  conglomerate  is  composed  of  less 
angular  fragments  and  more  varieties  of  lava  are  represented 
within  a  given  mass  of  the  conglomerate  than  in  the  breccia.  These 
differences  may  result  from  the  transportation  of  the  fragments  by 
running  water  for  limited  distances  down  the  slopes  of  the  volcano. 

Just  as  the  tuffs  furnish  the  most  indubitable  criteria  for 
establishing  the  volcanic  nature  of  the  series,  so  they  also  afford 
the  best  evidence  with  regard  to  the  topographical  features  of  this 
part,  at  least,  of  the  old  volcano.  Many  of  the  tuffs  seem  to  have 


THOROUGHFARE   VOLCANICS.  45 

originated  simply  as  sub-aerial  deposits  upon  the  gentle  slopes  of 
older  lava-flows,  but  in  other  cases,  the  deposition  may  have  been 
in  a  body  of  water.  The  sorting-  of  the  different  sizes  of  grain 
and  the  banding1  of  successive  beds  is  as  perfect  as  in  any  thin- 
bedded  shale  or  sandstone.  However,  the  fragments  of  minerals 
and  lavas  are  as  angular  as  in  the  sub-aerial  deposits,  so  that  they 
could  not  have  been  subjected  to  long  continued  wave-action.  On 
Iron  Point,  where  the  bedding  of  the  tuffs  is  best  exhibited,  the 
beds  of  finer  tuffs  are  often  eroded  on  their  upper  surfaces,  the 
succeeding  beds  showing  slight  local  unconformities.  Such 
erosion-breaks  in  the  beds  are  only  a  few  feet  in  extent  and  have 
resulted  probably  from  wave-action  along  the  tide-flats.  The  con- 
glomerate which  was  later  deposited  has  well-rounded  pebbles  and 
is  of  the  character  of  an  ordinary  sediment  which  thus  marks  an 
interlude  in  volcanic  activity,  although  the  conditions  do  not  seem 
to  have  been  favorable  for  marine  life,  as  was  the  case  when  the 
sediments  occurring  within  the  volcanic  rocks  on  Stimpson's  Island 
were  deposited. 

The  pyroclastics  and  other  sediments  show  that  this  volcanic 
area  was  near  the  sea-level  and  that  oscillations  of  level  were  more 
or  less  frequent  in  the  period  during  which  the  volcano  was  in 
eruption.  The  activity  did  not  close  with  the  eruption  of  the 
rocks  which  have  been  considered  as  the  North  Haven  series;  but 
in  the  later  phases  of  the  vulcanism,  the  products  were  of  a  some- 
what different  character  and  will  be  discussed  in  the  following 
section. 


VINAL  HAVEN  ACID  VOLCANICS. 

DISTRIBUTION.  The  area  of  the  acid  volcanics  may  be  describ- 
ed as  a  rough  circle  fitting-  into  the  crescent  of  more  basic  vol- 
canics, the  horns  of  which  extend  southward,  the  diameter  of  this 
nearly  circular  area  being-  about  two  miles.  Extremely  g-ood 
sections  of  the  series  are  afforded  by  Perry's  and  Crockett's  Coves 
which  extend  well  into  the  center  of  the  area.  The  topography, 
like  that  of  the  other  volcanics,  is  quite  bold,  especially  in  the  south- 
ern portion,  where  the  elevation  of  210  feet,  the  greatest  on  the  Fox 
Islands,  is  attained. 

DESCRIPTION  OF  ROCK-TYPES.  The  volcanic  rocks  of  this  area 
exhibit  less  variety  than  those  of  the  area  to  the  north.  Fewer 
types  are  to  be  distinguished  and  described,  yet  these  are  of 
special  petrographic  interest.  They  rank  among-  the  most  beauti- 
ful of  volcanic  rocks  and  cannot  fail  to  attract  attention  as  exposed 
on  the  weathered  ledg-es  and  sea-cliffs.  Among-  these  acid  vol- 
canics, both  lavas  and  pyroclastics  are  ag-ain  represented  and  four 
rock-types  will  be  described:  the  taxitic  and  the  spherulitic  apo- 
rhyolites,  the  flow-breccias,  and  the  tuffs. 

APORHYOLITES.  The  acid  lavas  which  form  the  greater  part  of 
this  Vinal  Haven  series  are  characterized  by  flow-structures.  No 
phenocrysts,  other  than  small  mag-netite  and  zircon  crystals,  ap- 
pear in  any  of  the  specimens  examined,  and  the  rock  is  commonly 
very  felsitic  in  appearance.  In  the  thin  section,  the  texture  is 
found  to  be  cryptocrystalline  in  part,  and  elsewhere  to  exhibit 
traces  of  glassy  structures,  now  more  or  less  obliterated  by  de- 
vitrification. In  short,  the  rock  is  rhyolitic  and  was  originally 
somewhat  glassy,  and,  therefore,  the  term  aporhyolite  has  been 
applied  to  the  type.  The  two  characteristic  structural  types  of 
these  aporhyolites,  which  may  be  distinguished  meg-ascopically, 
will  be  considered  separately. 

The  taxitic  aporhyolites  are  those  in  which  the  fluxion  charact- 

46 


VINAL   HAVEN    ACID   VOLCANICS.  47 

er  is  most  marked.  Flow-f  elsite  was  the  field-term  employed  in  de- 
scribing- these  rocks,  and  megascopically  as  well  as  microscopical- 
ly, this  lava  is  striking-  in  appearance.  It  is  very  compact  and 
composed  of  lig-ht  flesh-colored  and  dark  gray  or  purple  layers  of 
var}dng  width.  Where  this  banding-  is  regular,  the  rock  often 
shows  a  parallel  parting-.  There  are  many  cases  in  which  the 
flow-structure  is  very  irregular,  and  on  the  weathered  surface  of 
the  taxtites  the  structure  is  most  beautifully  exhibited,  the 
flowag-e-lines  being-  extremely  delicate.  In  the  thin  section,  the 
taxitic  structure  is  equally  apparent.  The  darker  lines  are  seen  to 
owe  their  color  to  masses  of  black  cumulitic  grains,  marg-arites, 
and  trichites,  minute  crystallites,  all  of  which,  doubtless,  have  the 
composition  of  magnetite,  being-  often  connected  with  grains  of 
that  mineral.  The  detail  of  the  fluctuation  as  presented  by  the 
arrang-ement  of  these  crystallites  is  much  more  delicate  than  that 
seen  megascopically. 

The  parallelism  in  the  more  regularly  banded  taxites  is  also 
well  shown  in  polarized  lig-ht,  by  the  varying-  degree  of  crystalliza- 
tion. The  bands  in  which  the  trichitic  flow-lines  are  so  promi- 
nent often  contain  chains  of  minute  spherulites,  which  will  be 
described  later  with  the  other  spherulites.  More  commonly,  how- 
ever, these  bands  are  characterized  by  a  peculiar  intergrowth  of 
fine  fibers  of  what  seem  to  be  quartz  and  feldspar,  althoug-h  the  con- 
stituents are  too  minute  to  be  determined.  This  intergrowth  may 
be  described  most  accurately  as  a  web-structure,  the  long-  delicate 
threads  being  more  continuous  in  the  direction  parallel  to  the 
megascopical  banding.  In  the  lighter  band  and  lenses,  the  crystal- 
lization is  coarser  and  less  definite  in  structure,  although  spheru- 
Jitic  in  part.  Some  areas  of  quartz  and  epidote  occur,  and  are 
doubtless  of  secondary  origin. 

The  spherulites  and  the  web-structure  seem  to  represent  inter- 
growths  of  mineral  fibers  which  crystallized  within  the  lava,  later 
than  the  trichites  and  other  crystallites,  but  before  consolidation. 
Their  parallel  arrangement  may  have  been  determined  by  chemi- 
cal or  physical  differences  along  definite  planes,  while  the  more 
intricate  flow-lines  traced  by  the  crystallites  are  the  result  of 
movement  in  the  lava.  In  certain  areas  in  these  rocks,  the 
crystallization  is  of  a  more  granular  character  and  these  rather  in- 


48  GEOLOGY   OF   THE   FOX   ISLANDS. 

definite  mosaics  of  quartz  and  feldspar  may  have  originated 
through  devitrification  of  original  glassy  portions  of  the  rock. 

The  spherulitic  aporhyolites  are  closely  related  to  the  taxitic, 
and  gradations  are  found  between  the  two  types.  The  spherulites 
vary  in  abundance  and  in  size,  a  few  small  ones,  the  size  of  a  pin- 
head  or  of  a  pea,  occurring-  disseminated  through  the  variegated 
purple  or  dark  gray  rock,  or,  on  the  other  hand,  the  spherulites 
being-  in  great  abundance,  often  completely  covering-  the  exposed 
surfaces,  and  attaining-  a  size  of  several  inches  in  diameter.  Such 
a  rock  is  of  so  unusual  an  appearance,  that  it  would  readily  be 
noticed  by  even  a  casual  observer.  The  spherulites  vary  greatly 
in  form,  some  being-  perfect  spheres,  others  ellipsoids,  and  often 
several  are  united  to  form  irreg-ular  nodules.  They  are  usually 
compact  and  felsitic,  more  rarely  hollow  and  lined  with  quartz. 
The  hollow  forms  may  owe  their  condition  to  subsequent  solution, 
and  no  undoubted  lithophysal  structures  were  observed  in  these 
old  rocks  which  have  so  many  characters  in  common  with  later 
rhyolitic  lavas. 

Examined  microscopically,  the  spherulitic  aporhyolite  is  found 
to  be  characterized  by  even  more  delicate  arrangements  of  the 
trichites  and  other  crystallites.  The  spherulites,  which  may  be 
described  as  complex  intergrowths  of  crystal-fibers,  radially  ar- 
ranged, vary  in  outward  form,  the  sections  being  circles,  segments 
of  circles,  and  plumes.  The  radiate  arrangement  of  the  fibers  is 
the  common  characteristic  and  the  different  outward  forms  taken 
by  such  intergrowths  are  equally  expressive  of  the  spherulitic 
crystallization,  as  has  been  shown  by  Prof.  Iddings1.  These 
spherulites,  especially  the  smaller,  are  not  always  to  be  seen  in 
ordinary  light,  as  they  are  often  wholly  independent  of  the  flow- 
structure,  the  lines  of  trichites  passing  through  the  spherulites 
without  interruption.  In  other  cases,  the  spherulites  may  be 
readily  seen,  through  a  concentric  arrangement  of  bands  rich 
and  poor  in  trichites;  or  again,  the  spherulitic  forms  may  be 
slightly  different  in  color,  the  finely  divided  pigment  being  of  a 
brown  tint  rather  than  black,  a  feature  noted  in  some  of  the 
spherulites  of  Obsidian  Cliff2.  The  micro-spherulites  and  the 

1.  Spherulitic  Crystallization.     Bull.  Phil.  Soc.  of  Washington,  vol.  n,  p.  459. 

2.  Iddings:     7th  Ann.  Report,  U.  S.  G.  S.,  p.  277. 


VINAL    HAVEN   ACID   VOLCANICS.  49 

chain-spherulites  found  both  in  this  rock  and  in  the  taxitic  apo- 
rhyolite  are  not  apparent  except  in  polarized  light.  In  the  chain- 
spherulites  there  is  an  approach  to  the  axiolitic  type,  and  it  is 
noted  that  a  group  of  three  black  crosses,  as  seen  when  the  axis  of 
the  chain  is  in  parallel  position  with  one  of  the  nicols,  resolves 
itself  into  two  crosses,  rather  than  three,  when  the  section  is  re- 
volved 45°.  These  micro-spherulites  often  compose  considerable 
portions  of  these  aporhyolites,  producing-  an  effect  similar  to  that 
pictured  by  Iddings1. 

The  constitution  of  these  spherulitic  intergrowths  is  a  problem 
for  the  solution  of  which  these  old  rocks  do  not  furnish  as  satis- 
factory material  as  the  younger  lavas.  Yet,  some  of  the  charact- 
eristics of  these  structural  forms  can  be  observed  in  these  apo- 
rhyolites and  deserve  description  as  affording-  some  degree  of  proof 
of  the  essential  similarity  of  the  old  lavas  to  their  later  equiva- 
lents. The  smaller  spherulites  are  optically  neg-ative;  while,  in 
the  larg-e  spherulites,  the  radiate  structure  can  be  resolved  into 
more  or  less  distinct  fibers,  the  optical  character  of  which  could 
not  be  determined,  owing-  to  the  fact  that  several  are  usually 
superimposed,  even  in  the  thin  section.  In  the  best  examples  of 
the  spherulitic  crystallization  on  the  larg-er  scale,  these  radiating 
fibers  are  seen  to  be  embedded  in  interlocking-  grains,  relatively 
larg-e;  and  the  radiate  structure  can  be  observed,  even  in  ordinary- 
lig-ht,  owing-  to  the  slig-ht  difference  in  refraction  and  in  color  be- 
tween the  fibers  and  the  grains.  These  grains  appear  to  be  uni- 
axial,  in  part  at  least,  and  most  probably  are  quartz,  while  the 
arborescent  fibers  are  feldspar.  Such  an  intergrowth  is  a  form  of 
the  micropegmatitic  structure,  and  is  undoubtedly  the  original 
form  of  these  larger  spherulites.  There  are,  however,  indications 
that  in  some  cases,  subsequent  alteration  has  replaced  the  feldspar 
fibers  with  quartz,  thus  changing  the  micropegmatitic  structure  to 
a  purely  granular  mosaic.  In  one  spherulite,  this  change,  al- 
though only  partial,  seemed  more  complete  in  the  outer  grains, 
where  only  mere  traces  of  the  fibers  remained,  while  nearer  the 
center,  the  arborescent  growth  was  continuous.  (Plate  I,  Fig.  6.) 
Where  the  change  has  been  complete,  the  spherulite  is  identified 
simply  from  the  circular  outline  of  the  fine  mosaic. 

I.     yth.  Ann.  Report,  Plate  17,  Fig.  i. 


50  GEOLOGY   OF  THE   FOX   ISLANDS. 

Different  generations  of  the  spherulitic  crystallization  are  in- 
dicated in  some  of  the  sections  examined.  The  spherulitic  plumes, 
where  they  occur  in  connection  with  the  more  perfect  spherulites, 
are  of  a  darker  color  and  appear  to  be  older,  being1  sometimes  en- 
closed in  the  later  spherulitic  growth,  or  conditioning-  the  outward 
form  of  the  larger  and  lighter  spherulite.  In  one  section,  all  of 
these  plumes  are  seen  to  have  grown  in  the  same  direction,  parallel 
with  the  flow,  and  also  to  have  determined  sharp  eddy-like  curves 
in  the  flow-lines  of  trichites.  One  of  the  most  beautiful  of  these 
spherulitic  structures  is  where  a  long-  plume  suddenly  spreads  out 
into  three  quadrants  of  a  circle,  as  seen  in  section. 

The  matrix  in  which  these  larg-er  spherulites  occur  is  usually 
cryptocrystalline,  although  there  are  areas  of  coarser  grains  of 
quartz  and  feldspar,  around  which  there  seems  to  be  some  flowag-e. 
These  may  represent  vesicles  in  the  rock,  subsequently  filled. 
Allied  with  these  are  smaller  miarolitic  areas  of  quartz  into  which 
project  fine  laths,  optically  continuous  with  the  fibers  of  small 
spherulites.  That  the  rock  was  in  part  g-lassy,  is  proven  by  the 
perlitic  cracks  which  are  beautifully  shown  in  some  of  the  sections. 
These  are  outlined  as  lig-hter  lines  in  the  ground-mass  of  a  lig-ht 
brown  color,  and  in  polarized  lig-ht  are  seen  to  have  been  filled 
with  quartz.  In  these  portions  of  the  rock,  the  very  delicate 
trichites  do  not  appear  to  have  been  affected  by  the  subsequent 
devitrification  of  the  glass. 

The  above  description  of  the  structures  which  are  so  character- 
istic of  the  taxitic  and  spherulitic  aporhyolites  of  Vinal  Haven  can 
express  only  imperfectly  the  fine  details  which  afford  to  the 
observer  the  most  conclusive  evidence  of  the  practical  identity  of 
these  rocks  with  recent  acid  lavas. 

An  analysis  of  one  of  the  most  typical  and  seeming-ly  least 
altered  spherulitic  aporhyolites  was  made  by  Messrs.  K.  W.  Ma- 
gruder  and  W.  A.  Jones,  and  it  affords  the  best  evidence  obtain- 
able, as  to  the  rlryolitic  character  of  the  magma  from  which  these 
spherulites  crystallized. 


VINAL    HAVEN    ACID   VOLCANICS.  51 

Si  O2  ,'.'-'.      -'v  :      ;      ,',Y  •:.>;*      .         .       77.28 

Ti  O2  ....         .         .         .  .         .    trace 

A12O3  \  >. .:'     11.24 

Fe2O3  >':,     .               _'•'•*.;;            '  .;        .     1.74 

Fe  O  .         .         .       ....       'v    >,  Y  '..-  .         none 

MgO  .               ".      '   .         .  .         .     0.21 

Ca  O  .         .         .         .. .       .  -      .         .         trace 

K2O  .                -..'.'.-       .         .  ,  \      .     4.55 

Na2O  £         .         .         .      ....        .         .          3.10 

p2o5  .    ,  .  ,  <^     v     . :  .      .   0.022 

H2O  (by  Pb  O)       .        V      .  .-         .         .  1.16 

CO2  .         .         .         .         .  .               0.17 


99.472 

FLOW-BRECCIAS.  Many  of  the  aporhyolites  present  traces  of 
brecciation.  Meg^ascopically,  the  rock  is  similar  to  the  spherulitic 
lava,  but  on  examination  is  found  to  lack  homogeneity.  In  some 
cases,  fragments  of  the  older  andesitic  lavas  are  included  with  those 
of  the  acid  g-lass,  but  the  matrix  is  seen  to  have  been  a  molten 
lava,  thus  making-  the  type  a  flow-breccia.  Under  the  microscope, 
the  mass  of  the  breccia  is  observed  to  be  characterized  by  a  flow- 
structure,  much  more  irregular  than  in  the  taxites.  One  specimen 
affords  a  beautiful  example  of  the  structures  seen  in  obsidians, 
ribbons  and  filaments  of  the  trichitic  g-auze  being-  twisted  into  com- 
plex patterns.  (Plate  I,  Fig-.  5.)  Traces  of  perlitic  cracking-  also 
give  evidence  of  the  originally  glassy  state  of  the  rock. 

The  fragments  which  are  similar  to  the  enclosing-  material 
usually  appear  to  have  been  brecciated  in  situ.  Spherulitic  plumes 
are  broken  across  and  the  parts  slig-htly  displaced,  and  in  all  cases 
the  spherulitic  crystallization  preceded  the  brecciation.  Other 
frag-ments  possess  a  flow-structure  sharply  contrasted  with  that  of 
the  enclosing-  lava,  and  in  one,  there  has  been  a  marked  shearing 
without  rupture1.  Devitrification  has  made  these  glassy  rocks 
quite  holocrystalline,  but  in  the  rock  with  the  delicate  obsidian 
structures,  there  has  been  very  slight  devitrification. 

TUFFS.     The  tuffs  of  this  series  are  in  the  main,   much   more 


i.     Figured  by  Dr.  Williams,  op.  cit,  p.  23. 


52  GEOLOGY   OF   THE   FOX   ISLANDS. 

compact  than  those  of  the  North  Haven  series,  but  like  them  vary 
in  color  from  dark  purple  to  light  pink.  In  grain,  they  present 
the  same  variation  from  a  coarse  breccia  to  a  rock  composed  of  the 
finest  particles  and  showing*  the  ash-structure.  The  larger  frag- 
ments  are  mostly  of  the  banded  and  spherulitic  lavas,  but  in  some 
instances,  andesite  fragments  are  very  plentiful.  Crystal  frag- 
ments also  occur  in  the  finer  tuffs. 

Epidote  occurs  in  these  tuffs,  partly  in  the  role  of  cement,  and 
partly  as  an  alteration  product.  This  mineral  was  also  found  in 
the  lavas  of  this  series,  but  in  less  amount.  Chlorite  and  calcite 
also  occur  as  secondary  minerals,  the  latter  only  in  tuffs  with 
andesite  fragments. 

SEQUENCE.  The  structure  of  the  Vinal  Haven  series  is  com- 
paratively simple,  the  lavas  and  tuffs  forming  a  basin,  which  over- 
lies the  tilted  andesitic  volcanics.  Where  their  bedding  can  be 
determined,  these  acid  volcanics  dip  at  an  angle  of  45°  or  over 
toward  the  center  of  the  area,  except  in  the  southwestern  part 
where  the  dip  is  more  to  the  southeast.  On  the  northwestern  side 
of  the  area,  the  lower  members  of  the  Vinal  Haven  series  are 
found  in  contact  with  the  more  basic  series,  and  the  relations  seem 
to  be  wholly  comformable.  To  the  northeast,  the  contact  is  hid- 
den, for  the  most  part,  by  the  dike  of  diabase. 

Tuffs  constitute  the  basal  members  of  the  series  and  the  separa- 
tion of  many  of  these  tuffs  from  those  of  the  underlying  series  is 
difficult.  Above  these  tuffs  the  general  sequence  is  taxitic  apo- 
rhyolite,  overlaid  by  the  spherulitic  aporhyolites  and  flow-breccias, 
with  the  other  tuffaceous  beds  as  probably  the  highest  members  of 
the  series  represented.  In  the  taxitic  lavas,  the  lamination  gives 
approximately  the  same  strike  and  dip  as  the  underlying  tuffs. 
The  spherulitic  aporhyolites  are  best  exposed  in  the  central  por- 
tion of  the  area,  on  the  shores  of  Perry's  and  Crockett's  Coves,  and 
especially  along  the  shore  south  of  Brown's  Head. 

CONDITIONS  OF  VOLCANIC  ACTIVITY.  The  volcanic  eruption  of 
these  acid  lavas  and  tuffs  immediately  succeeded  that  of  the  North 
Haven  series.  Indeed,  it  seems  probable  that  the  ejection  of  the 
acid  lava  began  before  that  of  the  andesitic  closed.  Within  the 


VINAL    HAVEN   ACID   VOLCANICS.  53 

North  Haven  series,  acid  members  were  found,  overlaid  by  more 
basic  types  of  lavas  and  pyroclastics,  and  again  in  the  upper 
tuffaceous  beds  of  what  has  been  termed  the  North  Haven  series, 
there  is  a  mixture  of  material.  Thus,  at  Brown's  Head,  a  coarse 
tuff-breccia  composed  of  large  fragments  of  andesites,  was  found  to 
contain  a  few  angular  pieces  of  the  taxitic  and  spherulitic  apo- 
rhyolites.  These  must  have  been  derived  from  some  lava-mass 
older  than  the  similar  lavas  now  exposed  to  the  east.  As  far  as  a 
difference  of  age  is  concerned,  the  separation  of  the  North  Haven 
and  Vinal  Haven  series  is  purely  arbitrary.  However,  it  seems 
probable  that  the  two  types  of  lavas  were  erupted  from  separate 
although  neighboring  vents,  the  rhyolitic  eruptions  beginning 
while  the  andesitic  were  in  their  later  stages. 

The  taxites  which  directly  overlie  the  deposits  of  fragmental 
volcanic  material,  are  a  striking  type  of  lavas.  The  lamination 
which  characterizes  them  is  expressive  of  the  conditions  of  their 
eruption  and  consolidation.  These  layers  differing  in  color  and  in 
degree  of  crystallization  have  resulted  from  the  flow  of  the  viscous 
lava,  spreading  over  a  surface  horizontal  or  only  slightly  inclined. 
Where  this  lamination  is  most  regular  and  the  layers  quite  thin,  a 
comparatively  greater  distance  from  the  point  of  origin  may  be 
inferred1. 

Loewenson-Lessing  in  calling  attention  to  this  taxitic  type  of 
lavas  has  considered  them  as  more  closely  allied  to  pyroclastics2. 
The  fact  of  the  existence  of  a  gradual  passage  between  the  lavas 
and  the  pyroclastics  may  well  be  emphasized  in  view  of  their 
common  origin,  yet  the  explanation  of  the  taxites  as  clastic  in 
character  seems  unwarranted.  A  simultaneous  disintegration  and 
cementation  within  a  molten  rock-mass  may  be  conceivable,  but  is 
hardly  sufficient  ground  for  fixing  the  clastic  character  of  a 
banded  lava.  Brecciation  can  take  place  in  an  already  consolidat- 
ed rock,  but  the  term  does  not  seem  applicable  in  describing 
differentiation  of  a  magma  into  layers  of  slightly  different  chemi- 
cal composition  and  physical  condition. 


1.  Iddings:      yth  Ann.  Report  U.  S.  G.  S.,  p.  260. 

2.  Tschermak's  Min.  u.  Petrog.  Mitth.  1888,  p.  528. 
Bull.  Soc   Beige  cl.  Geol.,  vol.  5,  p.  103,   1891. 


54  GEOLOGY   OF   THE   FOX   ISLANDS. 

The  macro-spherulites  first  appear  in  the  upper  part  of  this 
taxitic  aporhyolite,  where  they  are  confined  to  the  lighter  layers. 
The  conditions  determining-  this  kind  of  crystallization  are  not 
easily  understood.  The  arrangement  of  the  spherulitic  growths  in 
these  old  rhyolitic  lavas  point  to  the  fact  that  whatever  agent  con- 
tributed to  the  crystallization,  it  was  often  confined  to  quite 
definite  planes,  and  that,  in  some  cases,  the  conditions  favoring 
the  process  were  not  limited  to  a  single  period  in  the  consolidation 
of  the  lava.  Some  of  the  earlier  plumes  seem  to  have  been  formed 
before  the  lava  had  ceased  flowing-,  while,  in  other  cases,  the 
delicate  trichitic  lines  of  flow  maintain  the  same  course  within  the 
perfect  spherulites  as  in  the  rest  of  the  rock.  These  arborescent 
intergrowths  of  .quartz  and  feldspar  seem  to  have  crystallized  very 
rapidly  in  a  glassy  lava,  which  had  come  to  rest  and  become 
nearly  solid,  superheated  water  and  steam,  doubtless,  being  im- 
portant factors1. 

The  flow-breccias  appear  to  be  locally  developed  within  the 
central  area  of  spherulitic  aporhyolites.  In  part,  these  breccias 
probably  originated  as  lava  flows  including  fragments  of  older 
lavas,  as  was  the  case  in  the  breccia  so  prominently  developed  in 
the  North  Haven  series.  In  other  cases,  the  brecciation  seems  to 
have  been  confined  to  the  lava  itself,  certain  strains  to  which  the 
cooling  mass  was  subjected  being  sufficient  to  break  up  those  parts 
which  had  consolidated.  This  process  seems  to  have  been  of  a 
different  character  from  the  breaking  up  of  lenticles  of  flow  which 
would  give  an  ataxitic  or  rhyolitic  structure,  since  here  the  breccia- 
tion took  place  when  the  cooling  glass  was  sufficiently  stiff  to 
break  into  extremely  angular  fragments.  This  brecciation  also 
followed  the  spherulitic  crystallization  and  affords  the  best  proof 
of  the  original  nature  of  these  spherulites.  There  are  cases,  how- 
ever, in  which  the  dislocation  may  have  taken  place  in  the  la\*a 
when  in  a  less  viscous  state,  as  in  the  case  cited  above,  where  the 
flow-structure  shows  the  shearing.  A  similar  brecciation  in  a 
very  stiff  lava  has  been  observed  in  the  spherulitic  rock  of  Silver 
Cliff,  Colorado,  .where  the  spherulitic  crystallization  was  subse- 


I.     Cross:     Constitution  and  Origin  of  Spherulites.     Bull.   Phil.   Soc.   of  Washington, 
Aol.  ii,  p   433- 


THOROUGHFARE   VOLCANICS.  55 

quent  to  the  dislocation.  Cross1  considers  the  lava  to  have  been 
broken  by  sudden  and  violent  volcanic  shocks,  the  pieces  possess- 
ing- sufficient  plasticity  to  perfectly  coalesce  again.  In  the  Vinal 
Haven  occurrence,  a  greater  degree  of  plasticity  must  be  inferred 
in  many  cases,  since  the  separated  fragments  are  cemented  by 
glassy  material,  which  shows  fluidal  structures.  These  flow- 
breccias  are  especially  indicative  of  the  vicissitudes  in  the  process 
of  consolidation  of  a  lava-flow,  and  therefore  possess  more  than 
mere  petrographical  interest.  They  seem  to  be  a  type  of  volcanic 
products,  which  much  more  truly  occupy  the  middle  ground  be- 
tween lavas  and  pyroclastics  than  do  the  taxites,  to  which  refer- 
ence was  made  above. 

The  tuffs  occurring-  in  the  upper  part  of  the  Vinal  Haven  series 
differ  from  those  in  the  basal  portions  in  that  they  contain  no  frag- 
ments  of  the  andesitic  lavas.  Their  purely  acid  composition  in- 
dicates a  cessation  of  the  andesitic  eruptions,  which  opened  the 
Niag-ara  vulcanism.  How  much  more  of  rhyolitic  lavas  and  ejecta- 
menta  were  erupted  cannot  be  determined  owing-  to  the  granitic  in- 
trusion to  the  south  of  this  area  and  to  the  subsequent  erosion  of 
any  hig-her  members  of  the  series,  that  might  have  been  left  to  the 
north  of  the  granite  contact. 


:.     Op.  cit.,  p.  433. 


DIKES. 

DESCRIPTION.  As  one  walks  along-  the  shores  of  North  Haven 
and  of  the  northern  part  of  Vinal  Haven,  numerous  instrusive 
dikes  attract  attention.  These  may  stand  out  as  walls  from  the 
low  cliffs  of  less  resistant  rock,  or  more  often,  they  are  marked  by 
deep  cuts  extending-  into  the  bank.  In  the  interior,  the  dikes  are 
always  resistant  to  weathering-  processes,  and  thus  form  the  axes 
of  the  hills  and  ridges.  The  relative  resistance  of  country  rock 
and  dike  thus  varies,  but  whether  on  the  shore  or  inland,  these 
dikes  form  conspicuous  topographic  features. 

Of  these  dikes,  two  types  are  distinguished,  the  one  dark  and 
usually  finely  crystalline,  the  other  lig-ht  colored  and  felsitic  as  a 
rule,  althoug-h  in  a  few  cases  plainly  crystalline.  This  division 
will  be  seen  to  be  an  important  one  and  the  two  types  will  be  de- 
scribed separately. 

The  acid  dikes  rang-e  from  microgranites  or  quartz-porphyries, 
in  which  the  porphyritic  constituents  may  be  easily  distinguished, 
to  very  fine-grained  rocks  as  felsitic  in  appearance  as  the  acid 
lavas  of  Vinal  Haven.  The  coarser  of  the  quartz-porphyries 
might  be  mistaken  for  granites,  but  on  close  examination  it  is 
noticed  that  the  quartz  and  feldspar  occur  in  well  defined  crystals. 
Under  the  microscope  these  phenocrysts  are  seen  to  be  beautifully 
idiomorphic,  the  quartz  showing-  fine  examples  of  magmatic  cor- 
rosion. The  orthoclase  is  intergrown  with  albite,  and  plagioclase 
also  occurs  both  in  tabular  crystals  and  in  the  ground-mass. 
Muscovite  is  the  other  principal  constituent,  and  the  porphyry 
seems  rather  acid,  only  a  few  shreds  of  hornblende  being-  noted. 
The  ground-mass  in  the  coarser  type  is  microgranitic,  but  in  other 
of  the  quartz-porphyries,  it  shows  a  tendency  to  micropeg-matitic 
structure,  especially  as  borders  to  the  quartz  phenocrysts.  Pe- 
culiar sheaf -like  feldspars  occurring-  in  the  ground-mass  also  serve 
as  centers  for  such  intergrowths. 

The  felsitic  rock  is  of  a  light  green  color  and  is  found  to  have  a 

56 


DIKES.  57 

cryptocrystalline  ground-mass,  containing*  a  few  very  small  pheno- 
crysts  of  feldspar,  mostly  altered  to  calcite.  This  rock  occurs  in 
narrow  dikes  and  is  probably  the  equivalent  of  the  coarser  por- 
phyry of  the  larger  dikes.  All  of  these  acid  dike-rocks  are  of  a 
light  red  or  yellow  color  on  the  weathered  surface. 

The  basic  dike-rocks  also  show  considerable  range  in  grain. 
The  more  common  type  is  plainly  diabasic,  in  one  case  being*  very 
coarse-grained,  but  other  rocks  are  found  which  are  of  the  same 
dark  gray  color  and  wholly  aphanitic.  Examined  microscopically, 
the  former  type  is  found  to  be  composed  of  large  plates  of  augite, 
colorless  or  slightly  tinted  in  the  thin  section,  and  of  feldspar  laths 
of  an  earlier  crystallization  than  the  augite.  The  typical  diabase 
structure  is  modified  somewhat  in  a  few  specimens  by  a  porphy- 
ritic  development  of  the  feldspar.  Biotite  was  observed  in  some 
specimens,  but  only  in  small  amounts.  These  diabases  all  show 
more  or  less  alteration,  calcite  and  chlorite  being-  the  principal 
products.  *  t 

The  finer-grained  of  these  basic  rocks  are  to  be  grouped  with 
the  diabases  from  their  similarity  of  mineralogical  composition. 
The  diabase-structure,  however,  is  not  as  marked,  since  the  augite 
occurs  in  smaller  grains  while  the  mass  of  the  rock  is  a  felt  of  feld- 
spar laths.  A  few  larger  feldspars  occur  but  these  are  full  of  in- 
clusions and  badly  altered.  One  characteristic  of  this  type  is  the 
presence  of  quartz,  in  rounded  grains  relatively  very  large  as  com- 
pared with  the  other  constituents,  and  abundant  enough  to  be 
noticed  in  the  hand-specimen.  These  quartz  grains  are  surround- 
ed by  resorption-rims  of  augite  in  short  radiating  prisms,  partially 
altered  to  chlorite.  As  shown  by  Diller  in  his  study  of  the  quartz- 
basalt  of  California1,  the  presence  of  this  envelope  of  pyroxene  ex- 
presses only  the  composition  of  the  basic  magma  which  corroded 
the  quartz.  That  the  quartz  is  indigenous  to  the  diabase  which 
forms  these  dikes  seems  more  probable  also  from  the  wide  and  uni- 
form distribution  of  the  quartz  grains. 

DISTRIBUTION.  Throughout  these  islands,  the  basic  dikes  are 
more  numerous  than  those  of  acid  composition,  except  possibly  to 


Bull.  79,  U.  S.  G.  S.,  p.  28. 


58  GEOLOGY   OF   THE    FOX    ISLANDS. 

the  north  of  Mill  River,  where  the  interbedded  sheets  of  quart- 
porphyry  occur.  In  the  southern  part  of  North  Haven,  small 
dikes  of  the  diabasic  rocks  are  especially  common,  occurring"  as 
single  dikes  which  can  be  traced  for  long1  distances,  or  as  groups 
of  smaller  branching-  dikes.  No  system  could  be  traced  in  these 
dikes,  but  the  strike  is  commonly  between  northeast  and  north- 
west. The  acid  dikes  occurring-  on  North  Haven  are  also  usually 
small,  the  widest  being-  twenty  feet  in  width. 

On  Vinal  Haven,  the  dikes  are  much  larg-er,  those  of  the 
diabasic  character  being-  sufficiently  wide  to  be  mapped  in  two 
cases.  One  of  these  is  the  dike  which  extends  from  near  the  head 
of  Seal  Cove  north  to  Perry's  Cove  and  thence  probably  across  to 
Hopkins'  Point  and  along-  the  southern  shore  of  the  Thoroug-hfare. 
The  intrusive  nature  of  this  mass  of  diabase  is  proven  by  the  frag-- 
ments  of  the  overlying-  acid  volcanics  included  at  one  locality  and 
by  the  numerous  apophyses  which  cut  the  quartzitic  schists  to  the 
east.  In  its  northern  and  western  extension,  the  relations  are  not 
so  plain,  since  the  contacts  are  concealed,  but  the  manner  in  which 
this  rock  cuts  across  the  strike  of  the  well-bedded  tuffs  seems  to 
indicate  a  true  intrusion.  The  rock  in  this  dike  is  in  great  part 
that  characterized  by  the  quartz  grains,  althoug-h  to  the  south 
near  the  contact  with  the  granite,  it  is  a  typical  diabase.  It  is  in- 
teresting- to  note  that  the  same  quartz-bearing-  phase  occurs  in  the 
smaller  dikes  which  cut  the  volcanics  on  the  opposite  side  of  the 
Thoroughfare. 


DIABASIC  AND  GRANITIC  INTRUSIVES. 

AREAI,  DISTRIBUTION.  In  addition  to  the  dikes  of  igneous  rock 
which  have  been  injected  into  the  older  rocks,  there  are  larger  in- 
trusive masses  of  granite  and  other  holocrystalline  rocks.  These 
lie  to  the  south  of  all  of  the  areas  discussed  thus  far,  and  consti- 
tute the  greater  part  of  the  island  of  Vinal  Haven.  Of  these 
rocks  the  granite  is  the  more  important  in  areal  extent,  and  de- 
serves special  mention  also  from  the  fact  that  it  is  the  basis  of 
the  most  of  the  industry  of  the  southern  island. 

The  northern  ^  boundary  of  the  granite  area  is  a  line  running 
from  about  midway  on  the  eastern  shore  of  Calderwood's  Neck  due 
west  to  Seal  Cove  and  thence  southwest  to  the  northern  point  of 
Leadbetter's  Island.  To  the  south  of  this  contact  of  the  granite 
with  the  quartzitic  schists  and  acid  volcanics,  Vinal  Haven  is  com- 
posed wholly  of  granite,  with  the  exception  of  two  quite  large 
areas  of  darker  rock,  the  "black  granite"  of  the  quarry-men.  One 
of  these  areas  comprises  the  southern  extremities  of  the  main 
island  as  well  as  Lane  Island  and  part  of  Green  Island.  The 
other  and  the  larger  of  the  two  is  on  the  eastern  side  of  Vinal 
Haven  and  may  be  called  the  Barley  Hill  area,  from  the  more 
prominent  hill  within  its  limits.  The  western  boundary  of  this 
area  is  in  the  low  plain  occupying  the  central  portion  of  the  island, 
and  thus  the  contact  with  the  granite  is  hidden.1 

Topographically,  the  area  of  these  holocrystalline  intrusives  is 
characterized  by  the  same  rugged  shores  as  the  volcanic  areas. 
The  hills  are  mostly  bare  and  rounded  knobs  and  have  gentle 
slopes,  except  on  the  western  side  of  the  island,  where  hills  of  light 
granite  rise  from  the  shore  that  are  quite  imposing  as  one  sees 


i.  In  this  geological  study  of  the  Fox  Islands,  the  time  given  to  the  field-work  did  not 
permit  a  detailed  study  of  the  granite  area,  and  other  small  areas  of  the  diabase  or  diorite 
may  remain  unmapped.  It  is  believed,  however,  that  such  possible  omissions  will  not 
affect  the  geologic  relations,  as  here  presented. 

59 


60  GEOLOGY   OF   THE    FOX    ISLANDS. 

them  from  the  Bay.  The  central  portion  of  the  area  is  a  large 
plain,  swampy  in  part,  where  the  glacial  drift  is  doubtless  deeper 
than  over  any  other  portion  of  the  Fox  Islands. 

GRANITE.  The  principal  type  of  the  Vinal  Haven  granite  is  of 
a  light  gray  color,  with  a  slight  pink  tint,  and  of  a  rather  coarse 
texture,  but  very  uniform  both  in  color  and  in  grain.  The  con- 
stituents, which  are  prominent  in  the  hand-specimen,  are  the  clear 
quartz,  in  good  sized  areas,  the  feldspars,  white  and  flesh  colored, 
the  latter  being  more  abundant,  and  the  black  flakes  of  biotite. 
Near  its  contact  with  the  "black  granite"  the  granite  is  often 
rather  porphyritic,  the  feldspars  being  larger  and  more  idiomor- 
phic.  Another  type  of  the  granite,  which  is  found  on  the  eastern 
side  of  the  island;  is  fine-grained,  although  otherwise  similar  to 
the  principal  type. 

In  the  thin-section,  these  rocks  are  found  to  be  typical  biotite- 
granites,  with  the  normal  sequence  of  crystallization  of  their  con- 
stituents. The  orthoclase  is  more  abundant  than  the  plagioclase, 
shows  Carlsbad  twinning,  and  is  often  in  perthitic  intergrowths 
with  albite.  The  quartz  usually  occurs  in  large  allotriomorphic 
plates,  except  in  the  porphyritic  phase,  where  the  quartz 
blebs  are  rudely  idiomorphic,-  showing  an  interruption  in  their 
later  stages  by  the  crystallization  of  the  ground-mass.  Biotite 
occurs  in  large  plates  in  the  coarser  variety  and  in  small  shreds  in 
the  finer-grained.  Hornblende  is  an  accessory  constituent  in  vary- 
ing amounts,  although  in  the  porphyritic  variety,  it  is  in  almost 
equal  amounts  with  the  biotite  and  is  more  idiomorphic,  with  the 
usual  twinning.  Magnetite  and  apatite  are  the  other  constituents. 
The  feldspar  and  biotite  show  slight  traces  of  alteration,  but 
generally  the  rock  appears  very  fresh  and  shows  no  traces  of  hav- 
ing suffered  any  dynamic  action. 

The  granite  is  comparatively  free  from  the  segregation  patches 
of  dark  constituents;  but  within  the  granite  mass,  there  are 
numerous  aplitic  veins,  the  "salt-rock"  or  "salt-horse"  of  the 
quarrymen.  The  coarser  of  these  rocks  much  resembles  the  finer- 
grained  granite  in  general  appearance,  and  is  found  to  show  the 
same  texture  and  composition  except  that  hornblende  is  equally 
prominent  with  the  biotite,  intergrowths  of  the  minerals  occurring 


DIABASIC   AND    GRANITIC   INTRUSIVES.  61 

with  the  prism  of  the  former  parallel  with  the  basal  pinacoid  of 
the  latter.  Sphene  is  an  accessory  constituent  of  this  rock.  The 
finest-grained  of  these  granitic  veins  is  a  rock,  composed  of  a 
mosaic  of  rounded  grains  of  feldspar  and  quartz,  the  plagioclase 
in  a  few  cases  being-  somewhat  idiomorphic;  and  muscovite  occurs 
as  the  other  principal  constituent.  These  "veins"  are  of  the  class 
termed  "contemporaneous",  and  are  to  be  regarded  as  belonging-  to 
the  same  general  intrusion  as  the  rest  of  the  granite  mass,  rep- 
resenting material  injected  from  the  molten  portions  below  into 
fissures  in  the  already  consolidated  granite. 

This  granite  has  been  quarried  for  nearly  a  half-century;  and 
within  the  past  few  years,  the  Vinal  Haven  quarries  have  been  the 
most  extensive  in  operation  in  America.  The  principal  quarry  is 
the  Sands  Quarry  where  the  coarse-grained  granite  occurs.  This 
granite  is  also  quarried  at  numerous  points  on  Vinal  Haven  and 
the  smaller  islands,  the  most  important  of  which  is  Hurricane 
Island,  where  the  rock  is  especially  well  adapted  for  economical 
quarrying.  The  finer-grained  stone  is  quarried  at  the  East  Bos- 
ton and  Roberts  Harbor  quarries.  Paving-blocks  can  be  easily 
quarried  at  almost  any  point  in  the  granite  area,  and  numerous 
smaller  quarries  have  been  opened  for  this  purpose. 

The  value  of  this  granite  lies  in  the  easiness  with  which  it  is 
quarried  and  worked,  the  fine  polish  which  it  takes,  and  the 
proximity  of  the  quarries  to  good  harbors.  Mr.  G.  P.  Merrill, 
Curator  in  the  National  Museum,  illustrates  the  capabilities  of  the 
Vinal  Haven  quarries  by  the  statement  that  on  a  visit  to  these 
quarries  in  1883,  "he  was  shown  the  remains  of  a  huge  block  of 
granite  300  feet  long,  20  feet  wide,  and  varying  from  6  to  10  feet 
in  thickness,  that  had  been  loosened  from  the  quarry  in  a  single 
piece  and  afterward  broken  up.  The  largest  block  ever  quarried 
and  dressed  was  the  General  Wool  monument,  now  in  Troy,  New 
York,  which  measured  when  finished,  60  feet  in  height  by  5^  feet 
square  at  the  base,  or  only  6  feet  7  inches  shorter  than  the 
Egyptian  obelisk  now  in  Central  Park1." 

DIABASK  AND  DiORiTK.  The  "black  granite"  of  the  first  area 
mentioned,  that  forming  the  southern  part  of  Vinal  Haven,  is  an 

i     Stones  for  Building  and  Decoration,  1891,  p.   189. 


62  GEOLOGY   OF  THK   FOX   ISLANDS. 

olivine-diabase.  In  a  few  localities,  the  rock  is  coarse  and  almost 
gabbro-like,  the  plates  of  purple  augite  being*  equally  prominent 
with  the  feldspars.  Usually,  however,  the  diabase  is  fine-grained, 
dark  gray  when  fractured,  but  black  on  a  polished  surface.  Laths 
of  feldspar  can  be  distinguished  although  quite  small.  Micro- 
scopically, the  rock  is  seen  to  have  the  characteristic  diabase-tex- 
ture. The  feldspars  are  of  a  tabular  habit,  with  polysynthetic 
twinning*  by  the  albite  law  often  combined  with  the  pericline,  and 
the  tendency  to  idiomorphism  marked.  In  composition,  the  feld- 
spar is  generally  an  acid  labradorite.  Augite  of  a  faint  pink  tint 
is  the  constituent  second  in  importance,  occurring-  in  rather  large 
plates  enclosing-  the  feldspars.  Olivine  is  abundant  in  colorless, 
rounded  grains,  only  slig-htly  altered  and  is  the  oldest  of  the  es- 
sential constituents.  Mag-netite  occurs  in  irreg-ular  growth-forms 
and  aggregates  of  grains.  Small  amounts  of  biotite  are  found  as- 
sociated with  the  mag-netite  and  olivine,  and  also  some  green  horn- 
blende associated  with  the  aug-ite. 

In  the  Barley  Hill  area,  the  rock  is  of  a  slightly  different  char- 
acter. Megascopically  it  presents  phases  not  seen  in  the  other 
area,  and  in  the  thin  section,  the  differences  are  not  less  marked. 
In  short,  the  Barley  Hill  rock  shows  a  transition,  both  in  texture 
and  in  mineralogical  composition  from  a  diabase  to  a  diorite.  The 
type  nearest  the  olivine-diabase  just  described,  much  resembles  it 
in  the  hand-specimen,  although  somewhat  less  fresh  in  appear- 
ance and  slightly  finer-grained.  Examined  microscopically,  it 
shows  the  diabasic  structure,  although  less  perfectly  than  the  oliv- 
ine-diabase. Olivine  does  not  occur,  while  brown  hornblende  and 
biotite  are  equally  prominent  with  the  augite.  The  hornblende 
and  augite  occur  together  in  complex  intergrowths,  and  also  in 
separate  plates.  Biotite  is  usually  associated  with  the  magnetite, 
which  is  more  abundant  than  in  the  olivine-diabase. 

The  other  rocks  of  this  area  which  have  been  examined  are 
quite  different  megascopically,  being  more  feldspathic  and  thus 
lighter  in  color,  while  the  texture  is  more  granular  than  diabasic. 
Biotite  becomes  a  prominent  constituent.  In  the  thin  section,  the 
feldspar  is  seen  to  be  more  acid  than  in  the  diabasic  types,  and  the 
pyroxene  has  become  of  less  importance  as  a  constituent.  It 
occurs  in  intergrowths  with  the  biotite  and  hornblende  and  is 


DIABASIC    AND    GRANITIC    INTRUSIVES.  63 

apparently  older.  The  biotite  is  the  most  important  of  the 
ferro-mag-nesian  constituents  and  often  occurs  in  quite  idiomorphic 
plates.  Its  association  with  the  mag-netite  is  again  noticeable. 
The  green  hornblende  is  also  often  idiomorphic  in  the  prismatic 
zone,  althoug-h  irregular  shreds  also  occur.  Both  hornblende  and 
biotite  show  a  tendency  to  alter  to  chlorite.  Quartz  occurs  in 
relatively  small  amounts,  although  in  larg-e  irregular  areas,  being* 
the  last  constituent  to  crystallize.  Apatite  is  very  plentiful  in 
very  long-  slender  needles,  which  often  extend  throug-h  several 
different  constituents.  Titanite  is  found  associated  with  the 
biotite. 

The  intergrowths  of  aug-ite  and  hornblende  in  these  rocks  seem 
to  be  the  result  of  simultaneous  crystallization  of  the  two  minerals 
and  not  of  a  secondary  paramorphic  chang-e  of  the  pyroxene.  The 
fact  that  the  biotite  is  intergrown  with  the  aug-ite  in  a  similar 
way  shows  that  the  complexity  of  the  intergrowth  does  not  nec- 
essarily imply  the  derivation  of  the  hornblende  from  the  aug-ite. 
The  occurrence  of  idiomorphic  hornblende  is  sufficient  argument 
for  part,  at  least,  of  the  hornblende  being-  original.  The  view 
that  seems  sustained  by  the  microscopic  study  is  that  these  rocks 
represent  a  series  of  rock-types,  the  result  of  a  differentia- 
tion of  one  mag-ma.  One  end-member,  which  seems  to  characterize 
the  more  southern  area,  is  an  olivine-diabase,  while  the  Barley 
Hill  rock  is  principally  a  quartz-mica-diorite,  rich  in  hornblende 
and  containing-  varying-  amounts  of  aug-ite.  While  the  two  types 
were  easily  distinguished  in  the  field,  no  relations  were  observed 
other  than  those  of  transition  from  the  one  type  to  the  other. 

Both  the  diabase  and  the  diorite  have  been  quarried  to  some  ex- 
tent. The  hardness  of  the  olivine-diabase  interferes  with  its  g-en- 
eral  use,  yet  this  stone  is  well  fitted  for  monumental  purposes, 
since  it  is  of  uniform  color  and  takes  a  hig-h  polish.  Were  the  de- 
mand sufficient,  it  could  be  economically  worked  at  many  points 
along-  the  southern  shore  of  Vinal  Haven. 

RELATIVE  AGE.  Along-  the  shore  north  of  the  Reach,  there  is 
a  peculiar  ming-ling-  of  the  two  types  of  intrusives.  The  hills 
which  form  the  long-  peninsula  are  composed  of  olivine-diabase, 
but  anyone  standing-  on  the  deck  of  a  steamer  passing-  throug-h  the 


64  GEOLOGY    OF   THE    FOX    ISLANDS. 

Reach  cannot  fail  to  observe  that  this  dark  rock  is  striped  and 
checked  with  lighter  bands,  in  a  manner  almost  fantastic.  The 
diabase  is  cut  by  sheets  and  dikes  of  granite,  in  such  numbers  that 
the  darker  rock  almost  loses  its  continuity  in  this  labyrinth.  Both 
along  this  shore  and  at  places  on  the  western  hill,  the  old  contact 
surface  of  the  two  rocks  seems  to  have  been  just  reached  by 
erosion.  Some  of  the  larger  dikes  of  granite  can  be  traced  for  a 
considerable  distance  into  the  diabase  area,  and  at  the  contact,  the 
granite  is  of  a  porphyritic  character,  but  becomes  more  granular 
away  from  the  contact.  These  relations  afford  most  conclusive 
evidence  as  to  the  granite  being  a  later  intrusive  than  the  diabase. 
At  other  localities,  in  this  area,  as  well  as  in  the  Barley  Hill  area, 
the  relations  are  similar,  and  there  can  be  no  doubt  but  that  the 
granite  is  younger  than  the  diabase  and  diorite. 

RELATIONS  TO  DIKES.  The  general  mineralogical  similarity 
of  these  intrusives  to  the  two  types  of  dikes  is  suggestive.  In 
both  acid  and  basic  types  of  dikes  there  are  phases  which  approach 
structurally  as  well  as  mineralogically  the  granites  and  diabases  to 
the  south.  The  field  relations  also  fail  to  justify  a  sharp  distinc- 
tion between  the  two  occurrences.  In  the  case  of  the  acid  dikes  at 
the  head  of  Seal  Cove  and  of  Long  Cove,  the  connection  with  the 
granite  mass  can  be  traced,  and  a  similar  relation  with  the  quartz- 
porphyries  north  of  Mill  River  is  most  probable.  Wherever  the 
two  types  of  dikes  are  observed  in  contact,  the  acid  is  the  later; 
and  the  occurrence  west  of  the  mouth  of  Long  Cove,  where  the 
granite  cuts  the  large  diabase  dike,  also  confirms  the  hypothesis 
that  the  dikes  and  other  intrusives  are  connected  genetically,  the 
more  basic  type  preceding  in  time  the  granitic  intrusion,  with  its 
accompanying  acid  dikes. 

TIME  AND  CONDITIONS  OF  INTRUSION.  The  evidence  afforded 
by  the  apophyses  from  the  granite  mass  cutting  the  quartzitic 
schists  and  the  acid  volcanics  has  already  been  mentioned.  The 
younger  age  of  granite  has  also  been  shown  by  an  angular  frag- 
ment of  the  characteristic  acid  tuff  being  included  in  the  granite 
near  the  contact.  The  determination  of  the  age  of  the  diabase 
and  diorite  forming  the  large  areas  to  the  south  depends  upon  the 
correlation  of  these  with  the  basic  dikes.  According  to  this  view, 


DIABASIC    AND    GRANITIC    INTRUSIVES.  65 

the  diabasic   and  dioritic  intrusives  are  younger  than  the   rocks 
which  lie  to  the  north  of  the  granite  contact. 

Although  there  is  no  doubt  of  the  intrusive  character  of  the 
granite,  yet  a  definite  determination  of  the  age  of  the  intrusion  is 
somewhat  conjectural.  The  holocrystalline  character  of  the 
granite  is  usually  believed  to  express  slow  crystallization  under 
conditions  essentially  plutonic.  This  hypothesis  of  a  covering-  at 
the  time  of  the  intrusion  of  the  granite  magma  necessitates  the 
presence  of  thousands  of  feet  of  sediments  or  volcanic  deposits  of 
later  age  than  those  now  represented  on  these  islands.  The 
probability  of  the  tilting-  of  the  older  series  to  their  present  posi- 
tion before  the  granite  intrusion  is  also  to  be  considered.  The 
Calderwood's  Neck  schists  and  their  supposed  equivalents  to  the 
west  show  no  constant  relation  of  strike  and  dip  to  the  granite 
contact,  and  the  g-eolog-ic  structure  of  the  mass  of  acid  volcanics, 
so  far  as  it  is  evident,  does  not  seem  to  have  been  determined  by 
the  granitic  intrusion  but  rather  to  antedate  it.  Furthermore,  it 
is  probable  that  time  must  be  allowed  for  the  diabasic  intrusions 
between  the  eruptions  of  the  acid  lavas  and  the  intrusion  of  the 
granite,  and  the  former  intrusives  doubtless  also  consolidated 
beneath  a  considerable  thickness  of  rock.  Therefore,  if  the  sedi- 
mentary and  volcanic  rocks  described  on  the  preceeding  pages  are 
of  Niagara  and  pre-Niagara  age,  a  considerable  interval  of  post- 
Niagara  time  must  have  elapsed  before  the  intrusion  of  the 
granite. 


ALTERATION. 

CHARACTER  OF  ALTERATION.  The  Fox  Island  volcanics  have 
suffered  changes  since  their  consolidation  as  lavas  and  deposition 
as  tuffs,  and  to  explain  this  subseqent  history  is  one  of  the  phases 
of  the  geologic  problem.  In  the  long-  periods  of  time  since  their 
eruption,  these  volcanic  rocks  have  had  a  varied  history.  They 
have  been  buried  by  later  sediments,  cut  by  igneous  rocks  at  least 
two  distinct  times,  uplifted  by  mountain-making-  forces,  and  exposed 
to  atmospheric  agencies.  All  the  changes  which  express  the  effect 
of  these  varying-  conditions  may  be  considered  under  the  term  al- 
teration, which  thus  comprises  the  work  of  processes  acting-  under 
surface  conditions  and  those  of  greater  depth  and  less  superficial 
character;  that  is,  alteration  includes  weathering-  and  metamor- 
phism.1  Both  processes  have  contributed  to  the  final  result,  and  a 
sharp  line  of  separation  between  the  two  cannot  be  drawn,  and 
indeed  would  be  unnatural,  since,  in  nature,  there  is  perfect  transi- 
tion from  the  one  set  of  conditions  to  the  other. 

STRUCTURAL  AND  MINERALOGICAL.  The  chang-es  which  the 
rocks  of  the  Fox  Islands  have  suffered  are  of  two  kinds,  structural 
and  mineralog-ical.  The  former  is  of  a  metamorphic  character  and 
referable  to  dynamic  action  which  caused  a  crushing-  of  the  rock. 
Dynamic  metamorphism  has  been  important  only  in  the  case  of 
the  old  diabase  flows  which  have  been  converted  into  the  green- 
stone schists  of  North  Haven.  Here  the  effects  are  the  production 
of  a  schistose  matrix  around  columns  of  compact  rock,  and  the 
fracture  of  the  mineral  constituents.  The  absence  of  any  similar 
dynamic  metamorphism  in  the  case  of  the  younger  rocks  is  known 
through  a  vary  delicate  test.  The  quartz  phenocrysts  in  the 
quartz -porphyries,  which  occur  both  in  the  volcanic  series  and  as 
later  intrusives,  show  not  the  least  trace  of  peripheral  granulation 


i.     This  use  of  terms  is  as  given  by  Dr.  Williams  in  his  lectures  upon  metamorphism,  in 
1894  at  this  University.     See  also  Bull.  62,  U.  S.  Geol.  Survey,  p.  36,  1890. 

66 


ALTERATION.  67 

or  even  of  undulatory  extinction.  So  brittle  is  this  mineral  that  it 
is  not  possible  to  believe  that  these  quartz-porphyries  have  been 
subjected  to  the  same  strains  as  the  greenstones  to  the  north. 

The  mineralogical  changes  in  both  the  older  diabase  flows  and 
the  later  volcanic  series  are  much  more  important  than  the  structur- 
al changes  just  mentioned.  In  many  of  the  rocks  examined,  these 
changes  in  mineral  composition  have  been  so  complete  that  none  of 
the  constituents  have  escaped  destruction.  In  general,  these 
changes  have  been  of  the  character  of  pseudomorphism,  so  that  al- 
though the  mineralogical  composition  of  the  rock  has  been  much 
modified,  its  structure  or  texture  remains.  These  molecular  re- 
arrangements, by  which  certain  minerals  are  replaced  by  others, 
are,  in  great  part,  such  as  are  believed  to  result  from  processes 
that  are  of  the  nature  of  weathering  rather  than  of  metamorphism. 
Even  in  the  greenstone  it  is  seen  that  the  degree  of  mineralogical 
change  is  not  dependent  upon  dynamic  action,  while  no  cases  are 
known  oi  purely  contact  metamorphism,  except  in  the  Calder- 
wood's  Neck  series,  as  mentioned  on  page  29. 

For  convenience  in  treatment,  the  mineralogical  changes  may 
be  discussed  as  they  are  exhibited  in  the  alteration  of  the  feld- 
spathic  and  of  the  ferro-magnesian  constituents. 

The  mineralogical  alteration  of  the  feldspars  of  the  green- 
stone has  been  of  the  character  of  saussuritization.  In  this 
process,  aggregates  of  clear  zoisite  and  secondary  feldspar  grains 
have  replaced  the  original  feldspar  laths.  In  a  few  cases,  epidote 
is  present,  but  as  shown  by  Cathrein1,  the  epidotization  of  feld- 
spar differs  from  the  more  common  type  of  saussuritization  only  in 
the  greater  amount  of  iron  required. 

In  the  andesitic  lavas,  epidotization  has  been  quite  as  im- 
portant as  the  production  of  zoisite.  Large  areas  of  the  latter 
mineral  were  observed,  however,  and  the  relations  seem  to  in- 
dicate its  derivation  from  the  feldspar.  Calcite  also  occurs  as  one 
of  the  alteration-products  of  the  feldspar,  being  usually  found 
within  the  altered  crystal.  In  the  green  schists,  on  the  other 
hand,  although  the  calcite  undoubtedly  was  derived  from  the  feld- 
spar in  great  measure,  it  usually  filled  amygdules  or  occurred  in 


i     Zeitschr.  f.  Kryst.,  vol.  7,  p.  234.  1883. 


68  GEOLOGY   OF   THE    FOX    ISLANDS. 

irregular  areas.  The  most  striking-  case  of  the  alteration  of  feld- 
spar is  in  the  andesite  described  on  page  32.  In  this  rock,  the 
labradorite  phenocrysts  often  show  altered  cores,  the  alteration 
being-  to  an  aggregate  of  muscovite,  chlorite  and  calcite,  and  the 
boundary  between  this  core  and  the  unaltered  border  is  a  sharp 
line.  (Plate  I,  Fig-.  3.)  Such  a  zonal  relation  can  be  explained 
as  resulting-  from  a  difference,  either  physical  or  chemical, 
between  the  core  and  the  rim.  The  alteration  would  be  favored  in 
the  former  case  by  the  presence  of  inclusions  in  the  core,  and  in 
the  latter,  by  the  more  basic  character  of  the  core.  Comparison 
with  other  feldspar  phenocrysts  in  this  rock  shows  that  they  are 
more  basic  in  the  interior,  and  while  inclusions  are  present,  they 
occur  only  near  the  outer  rim.  Therefore,  this  zonal  alteration  is 
believed  to  have  originated  from  differences  in  chemical  composi- 
tion, the  more  basic  core  being-  more  easily  attacked  by  solutions1. 
The  ferro-mag-nesian  constituents  of  these  rocks  show  one 
principal  kind  of  alteration,  that  of  chloritization.  Some  small 
amounts  of  what  appeared  to  be  actinolitic  fibers  were  observed  in 
the  tuffaceous  greenstone;  but  the  absence  of  any  considerable 
amount  of  secondary  hornblende  in  these  pyroxene-bearing-  rocks  is 
a  remarkable  feature.  Such  uralitization  has  been  considered  a 
truly  metamorphic  process,  while  the  hydrous  chlorite  minerals 
are  rather  the  products  of  weathering-2.  The  chlorite  has  the 
optical  characteristics  of  pennine  and  shows  different  modes  of  de- 
rivation from  the  augite.  In  the  older  green  schists,  the  former 
presence  of  the  pyroxene  is  only  implied  from  the  shape  of  the 
triangular  areas  of  chlorite  between  the  feldspar  laths;  but  in  the 
andesites  and  diabase-porphyries  of  the  later  series,  the  chlorite 
forms  pseudomorphs  after  the  pyroxene  phenocrysts  or  occurs  in 
association  with  the  partially  altered  augite.  In  the  latter  case, 
the  progress  of  the  mineralogical  change  is  seen  to  be  along  de- 
finite planes  in  the  augite,  especially  the  orthopinacoid  (100), 
which  is  the  normal  solution  plane3.  It  may  be  added  that  a 


1.  Similar  cases  have  been  described  by  various  writers;    vid.     Grooser  :     Tsch.   Min. 
u.  Petrog.  Mitth.,  vol.  13,  p.  13;  and  Matthew:     Trans.  N.  Y.  Acad.  Sci.,  vol.  14,  p.   212. 

2.  Williams:  Bull.  62,  U.  S.  Geol.  Survey,  pp.  52-56. 

3.  Judd:   Min.  Mag.,  vol.  9,  p.  192.      1890. 


ALTERATION.  69 

similar  relation  to  definite  crystallographic  planes  was  noted  in  the 
arrangement  of  some  peg's  of  epidote  in  an  altered  feldspar  crystal1. 

The  alteration  of  the  ores  is  most  apparent  in  the  large  amounts 
of  leucoxene  present  in  many  of  these  rocks.  To  a  great  extent,  this 
leucoxene  appears  to  be  titanite  and  its  origin  from  ilmenite  is 
shown  most  beautifully  in  some  of  the  diabase-porphyries.  The 
titanite  and  rutile  found  in  the  greenstones  are  probably  derived 
from  ilmenite. 

Certain  apparent  irreg-ularities  in  the  processes  of  alteration  de- 
serve mention.  In  the  greenstones,  the  mineralogical  chang-es 
were  less  marked  in  the  rocks  which1  had  suffered  most  structural 
metamorphism;  the  feldspars  have  been  crushed,  but  yet  are  clear 
and  show  no  development  of  zoisite.  In  the  North  Haven  vol- 
canics,  specimens  which  had  their  feldspars  completly  altered, 
often  contained  augite  crystals  barely  affected,  and  vice  versa. 
These  facts  prove  that  the  mineralogical  changes  are  not 
necessarily  of  the  nature  of  reactions  between  the  components  of 
the  different  constituents. 

DEVITRIFICATION.  In  the  glassy  lavas,  the  process  of  devit- 
rification is  the  pre-eminent  mode  of  alteration.  The  presence  of 
granular  ag-greg-ates  of  crystalline  material  in  portions  of  the  rock, 
that  also  show  structures  believed  to  be  characteristic  of  glass, 
points  to  some  chang-e  having-  taken  place.  Devitrification  is  the 
process  that  will  account  for  this  chang-e  of  amorphous  g-lass  to  de- 
finite crystallized  minerals,  and  it  is  recog-nized,  therefore,  as  of 
chief  importance  in  the  explanation  of  the  differences  between  the 
old  rhyolites  and  their  recent  equivalents.  The  fact  that  these 
g-lassy  lavas  contain  spherulites  and  other  crystalline  structures 
that  are  undoubtedly  original,  makes  it  a  difficult  problem  to  de- 
cide to  just  what  extent  devitrification  has  contributed  to  the  pre- 
sent rock-structure.  The  absence  of  any  subsequent  dynamic 
action  in  these  lavas,  however,  favors  the  separation  of  primary 
and  secondary  crystallization.  This  feature  also  shows  that 
dynamic  action  is  not  needed  to  initiate  the  processes  of  devitri- 
fication. 


i.     Gentil  believes  that  the  alteration  of  nepheline   proceeds  in  a   similar  way  along  di- 
rections of  feeble  cohesion.     Bull.    Soc.  Franc,  d.  Min.,  vol.  17,  p.  108. 


70  GEOLOGY   OF   THE    FOX    ISLANDS. 

NOMENCLATURE  OF  ALTERED  IGNEOUS  ROCKS.  In  view  of  the 
changes  which  these  volcanic  rocks  have  undergone  since  their 
consolidation,  questions  of  classification  and  terminology  naturally 
arise.  The  usage  in  the  nomenclature  of  altered  igneous  rocks 
has  been  varied  and  in  the  case  of  the  volcanic  rocks,  it  has  been 
such  as  to  cause  the  true  nature  of  the  older  rocks  to  be  misin- 
terpreted. Present  and  acquired  differences  have  been  so  magni- 
fied as  to  be  regarded  as  genetic  and  inherent.  Thus  "older"  and 
"younger"  volcanic  rocks  have  been  separated  as  fundamentally 
distinct. 

In  the  present  study,  the  usage  adopted  has  been  that  of  re- 
taining the  name  of  the  equivalent  recent  volcanic  rock  for  the 
Paleozoic  lava.  Genetic  identity  has  been  recognized  in  this 
manner,  and  the  geologic  relations  interpreted  accordingly.  How- 
ever, it  is  realized  fully  that  these  old  andesites  and  rhyolites  are 
not  at  present  exactly  similar  to  the  Tertiary  andesites  and 
rhyolites.  It  is  doubtful  if  these,  differences  are  of  classificatory 
value,  yet,  an  ideal  nomenclature  should  give  recognition  to 
secondary  changes  which  have  contributed  so  much  to  the  present 
character  of  the  rock.  In  the  study  of  similar  old  volcanics  in  the 
South  Mountain  area,  Miss  Bascom1  suggested  a  terminology  for 
the  acid  volcanic  rocks,  in  which  devitrification  had  been  the 
process  of  alteration,  and  her  term,  aporhyolite,  has  been  used  in 
the  preceeding  pages  to  signify  a  devitrified  rhyolite. 

In  the  case  of  the  North  Haven  andesites  and  porphyries,  there 
seems  to  be  a  need  of  some  terms,  which  will  express  the  altera- 
tion of  these  rocks,  without  affecting  their  position  in  a  genetic 
classification.  In  the  present  instance,  the  alteration  has  been  not 
structural,  but  mineralogical  or  pseudomorphic.  Thus,  the  origi- 
nal nature  of  the  rock  can  be  determined  and  deserves  full  recogni- 
tion. Refinement  in  such  a  terminology  is  to  be  avoided,  since  it 
has  been  observed  in  these  volcanics  that  the  degree  of  alteration 
may  vary  greatly  within  a  small  area,  and  the  recognition  of  these 
different  degrees  of  alteration  would  be  impracticable.  Moreover, 
secondary  differences  are  not  to  be  emphasized,  since  they  are  not 
always  essential  to  the  interpretation  of  geologic  history.  How- 


i.     Op.  cit.,  p.  828. 


ALTERATION.  71 

ever,  it  seems  certain  that  the  ideal  nomenclature  of  altered  ig-- 
neous  rocks  has  not  been  secured;  and  it  is  with  the  hope  of  calling- 
attention  to  this  feature  in  the  study  of  ancient  volcanics  that 
these  pag-es  have  been  devoted  to  the  subject  of  alteration. 


GEOLOGIC  HISTORY. 

ACADIAN  TROUGH.  The  correlation  and  interpretation  of  any 
rock  series  involves  a  consideration  of  the  broader  topographical 
features  during-  the  period  in  which  these  rocks  may  have  been 
formed.  Such  a  view  must  comprise  the  whole  geologic  province 
of  which  the  special  area  is  a  part;  and  thus  it  becomes  necessary 
to  consider  briefly  the  Paleozoic  features  of  what  is  now  the  coast- 
region  of  Maine.  In  his  papers  on  the  Archaean  axes  of  North 
America,  Prof.  Dana  outlined  the  features  of  this  area,  at  the  be- 
ginning of  the  Paleozoic1.  The  Mount  Desert  Range  with  a  south- 
western trend  marked  the  old  coast-line,  which,  thus,  approximate- 
ly coincides  with  that  of  to-day.  To  the  southeast  lay  a  great 
basin  of  sedimentation,  the  Acadian  trough,  which  extended  from 
Northern  Newfoundland  to  the  present  Bay  of  Fundy  and  thence 
to  Massachusetts  and  Rhode  Island.  This  was  probably  not  an 
open  sea,  but  one  protected  by  barriers,  now  represented  by  Nova 
Scotia  and  the  more  shallow  parts  of  the  present  sea-floor,  which 
mark  the  southern  boundary  of  the  Gulf  of  Maine.  In  this  trough, 
sediments  were  deposited  throughout  Paleozoic  time  and  in  its 
various  portions  great  thicknesses  of  sedimentary  rocks  are  ex- 
posed to-day. 

PKE-NIAGARA  VOLCANIC  ACTIVITY.  Along  the  border  of  such 
an  area  of  great  sedimentation,  volcanic  outbreaks  might  naturally 
be  expected.  In  Southern  New  Brunswick,  the  pre-Cambrian 
rocks  have  been  found  to  be  made  up,  in  large  part,  of  surface 
volcanics,  lavas,  breccias,  and  tuffs,  petrographical  description  of 
which  has  been  published  by  Dr.  W.  D.  Matthew2.  Throughout 


1.  Bull.  Geol.  Soc.  Am.,  vol.  7,  p.  36. 

Am.  Jour.  Sci.  (3rd  series),  vol.  39,  p.  378. 

2.  Trans.  N.  Y.  Acad.  Sci.,  vol.  14,  p.  187.     1895. 
Bull.  Nat.  Hist.  Soc.  New  Brunswick,  No.  13.     1895. 


72 


GEOLOGIC    HISTORY.  73 

all  of  the  maritime  districts,  the  presence  of  such  volcanics  has 
been  recognized  by  the  Canadian  geologists1.  On  the  Maine 
coast,  the  volcanic  deposits  described  by  Professors  Shaler  and 
Davis  are  considered  to  be  younger,  of  Silurian  and  Devonian  age. 
Pre-Niagara  volcanic  activity  was  a  factor  also,  in  the  geologic 
history  of  the  Fox  Islands.  The  greenstones  forming  the  north- 
ern portion  of  North  Haven  originated  as  amygdaloidal  flows  of 
diabase  and  accompanying  deposits  of  coarse  and  fine  ejectamenta. 
The  lava-flows  were  characterized  by  contraction  phenomena 
similar  to  those  observed  in  the  lavas  of  Vesuvius  and  other  mod- 
ern volcanoes.  These  old  lavas  and  tuffs  are  metamorphosed  to 
such  an  extent  that  their  original  characters  are  greatly  obscured. 
This  relatively  greater  degree  of  metamorphism  seems  to  indicate 
dynamic  action  during  the  time-interval  between  the  eruption  of 
these  lavas  and  the  post-Niagara  volcanics,  which  do  not  show  any 
structural  metamorphism.  It  seems  probable  that  further  geologi- 
cal study  will  reveal  the  presence  of  other  areas  of  these  older 
volcanic  rocks  on  the  Maine  coast. 

NIAGARA  SEDIMENTATION.  Throughout  these  periods  of 
vulcanism,  sedimentation  doubtless  continued  in  adjoining  por- 
tions of  the  Acadian  trough.  Indeed,  on  account  of  the  loose 
nature  of  many  of  their  products,  the  volcanoes  would  furnish 
abundant  material  to  be  easily  eroded  and  transported  to  the 
Acadian  Sea.  In  the  area  under  discussion,  the  Calderwood's 
Neck  schists  represent  a  thick  series  of  arenaceous  and  argillaceous 
sediments,  which  were  deposited,  doubtless,  in  early  Paleozoic 
time,  but  their  exact  age  has  not  been  determined  through  lack  of 
fossils.  Similar  old  sediments  have  been  described  by  Shaler  as 
occurring  on  Mount  Desert2. 

Sediments  of  Niagara  age  occur  on  North  Haven  and  their 
varied  character  and  rich  fauna  doubtless  point  to  their  being 
younger  than  the  metamorphosed  schists  on  Calderwood's  Neck. 
The  basal  members  of  this  Niagara  series  were  deposited  upon  the 
diabase  flows,  which  were  evidently  considerably  disintegrated  on 
their  surface,  before  this  transgression  of  the  sea  in  Niagara  time. 

1.  For  review  of  literature  :     Williams:  op.  cit.,  pp.  14-21. 

2.  Op.  cit.,  p.  1037. 


74  GEOLOGY    OF    THE    FOX    ISLANDS. 

POST-NIAGARA  VOLCANIC  ACTIVITYC  While  the  six  hundred 
feet  of  Niagara  sediments  represent  an  interlude  in  the  volcanic 
activity  at  this  point  on  the  border  of  the  Acadian  trough,  the 
break  was  not  one  of  long  duration.  Even  within  the  Niagara 
series,  there  are  indications  that  abundant  fine  volcanic  dust  fell 
into  the  quiet  waters  and  were  deposited  forming  the  red  shale 
which  occurs  at  several  horizons  in  the  series.  Such  deposits 
simply  mark  the  commencement  of  a  period  of  vulcanism  which  has 
left  a  full  record  of  its  nature. 

Flows  and  streams  of  andesites,  andesite-porphyries,  and 
diabase-porphyries;  beds  of  volcanic  blocks,  more  or  less  rounded 
and  cemented  with  fine  dust  or  volcanic  mud  or  even  molten  lava 
of  later  eruption;  deposits  of  the  fine  volcanic  sand  and  dust  that 
fell  on  the  slopes  of  the  volcano  or  in  the  shallow  waters  of  the  ad- 
joining sea;  such  are  the  elements  that  constitute  the  record  of  the 
earlier  part  of  the  volcanic  outbreak. 

Later,  the  volcanic  vent  or  vents,  from  which  these  lavas, 
breccias,  and  tuffs  of  the  North  Haven  series  had  been  erupted, 
became  less  important.  Already  in  the  upper  portions  of  the 
series,  there  were  indications  of  an  intermittent  character  in  the 
eruptions,  since  several  beds  of  sediments,  not  purely  pyroclastic, 
occur  between  the  purely  volcanic  members.  Other  volcanic  pro- 
ducts of  a  decidedly  more  acid  composition  were  next  erupted,  pro- 
bably from  different  centers.  These  acid  volcanics  had  their  fore- 
runners, the  quartz-porphyries  occurring  within  the  North  Haven 
series.  Now,  however,  the  acid  rock-types  became  more  import- 
ant, and  rhyolitic  lavas,  taxitic  and  spherulitic,  were  erupted  and 
spread  over  the  surface.  Fragmental  material  was  also  ejected 
and  deposited  in  well-defined  beds. 

In  both  its  phases,  this  volcanic  activity,  that  directly  followed 
the  deposition  of  Niagara  sediments,  was  essentiall}7  similar  to  the 
vulcanism  of  today.  In  view  of  the  vicissitudes  of  their  sub- 
sequent history,  it  is  indeed  remarkable  that  these  old  volcanics  so 
well  express  the  conditions  of  the  volcanic  activity  during  Paleo- 
zoic time. 

What  was  the  extent  of  the  volcanic  area,  it  is  difficult  to  state. 
Study  of  the  Fox  Islands  rocks  points  to  the  fact  that  only  a  small 
part  of  the  volcano  or  group  of  volcanoes  is  represented  in  these 


GEOLOGIC    HISTORY.  75 

islands.  Correlation  with  the  volcanic  series  of  Mount  Desert 
would  be  highly  conjectural,  but  careful  g-eolog-ic  study  of  the  whole 
Penobscot  Bay  area  may  afford  data  for  a  more  complete  determin- 
ation of  the  limits  of  this  volcanic  group.  It  has  been  the  purpose 
of  the  present  study  to  contribute  to  such  a  history  of  the  Paleo- 
zoic vulcanism  of  this  portion  of  Maine  coast. 

DEVONIAN  INTRUSIONS.  At  some  time  after  the  close  of  the 
volcanic  activity  represented  in  the  rocks  exposed  on  the  Fox 
Islands,  there  was  a  tilting*  of  the  series,  with  some  folding"  and 
faulting-.  There  is  no  evidence  that  the  Niag-ara  rocks  were  tilted 
before  the  eruption  of  the  overlying-  volcanics,  but  there  are  indi- 
cations in  the  relations  of  the  acid  volcanics  to  the  granite,  that 
this  movement  antedated  the  granite  intrusion.  There  is  no 
further  evidence  bearing-  directly  upon  the  time  of  the  intrusions 
of  the  granite,  diabase,  and  diorite.  In  other  parts  of  the 
Acadian  province,  however,  the  great  granite  intrusions  of  Nova 
Scotia  and  New  Brunswick  belong-  to  the  close  of  the  Devonian1. 
Such  an  ag-e  for  the  Vinal  Haven  granite-mass  would  afford  an 
interval  for  the  deposition  of  a  cover  of  sediments,  sufficiently 
thick  to  assure  granular  crystallization  of  the  granitic  and  the 
more  basic  mag-mas;  and  therefore,  these  intrusions  are  considered, 
provisionally,  as  of  Devonian  ag-e. 

The  character  of  these  intrusions  seems  to  have  been  of  the 
nature  of  a  displacement  of  the  older  rocks  on  one  side  of  a  definite 
line  of  fracture.  Absorption  or  fusion  of  the  country-rock  by  the 
granite  is  not  at  all  probable,  since  at  the  contact,  the  granite 
shows  no  endomorphic  effects  other  than  a  slig-ht  chang-e  in  tex- 
ture. Study  of  the  granite  contact  with  the  quartzitic  schists  and 
acid  volcanics  sug-g-ests  that  the  intrusion  was  accomplished  by  the 
movement  of  the  rock  that  lay  to  the  south  of  the  fracture  line,  a 
displacement  which  may  have  been  horizontal,  or  in  part  vertical, 
and  thus  of  the  nature  of  a  laccolitic  intrusion,  the  unsymmetrical 
side  being-  to  the  north.  Althoug-h  the  exact  nature  of  this  in- 
trusion is  a  matter  of  conjecture,  the  true  intrusive  character  of 
the  granite  and  other  holocry stall ine  rocks  seems  well  established. 


J.  W.  Dawson :  Quart.  Journ.  Geol.  Soc.,  vol.  44,. p.  814. 


76  GEOLOGY    OF    THE    FOX    ISLANDS. 

Thus,  following*  the  eruption  of  the  volcanics  of  basic  and  acid 
composition,  there  was  a  similar  sequence  of  basic  and  acid  plu- 
tonics. 

The  g-eologic  history  of  the  Fox  Islands,  so  far  as  rock-making- 
is  concerned,  closed  with  the  intrusion  of  the  granite.  The  later 
history  has  been  that  of  destruction  and  degradation  of  the  differ- 
ent rocks,  and  the  carving-  of  the  rock-masses  to  their  present  re- 
lief, by  atmospheric  processes. 


LIFE. 

George  Otis  Smith  was  born  in  Hodg-don,  Maine,  February  22, 
1871.  His  early  education  was  obtained  in  the  public  schools  of 
Augusta  and  Skowheg-an,  Maine,  and  in  1889,  he  entered  Colby 
University,  from  which  institution  he  was  graduated  in  1893,  with 
the  degree  of  A.  B.  In  the  three  years  following,  he  has  pursued 
graduate  courses  of  study  at  Johns  Hopkins  University  in  Geology 
under  Professors  Williams  and  Clark,  Dr.  Mathews  and  Messrs.  G. 
K.  Gilbert  and  Bailey  Willis,  and  in  Chemistry  under  Professors 
Remsen  and  Renouf.  He  has  held  the  University  Scholarship  in 
Geology,  1894-5,  and  the  Fellowship,  1895-6..  During-  the  field- 
seasons  of  1893,  1894,  and  1895,  he  has  been  eng-aged  in  g-eological 
work  in  the  States  of  Michigan  and  Washington  in  the  employ  of 
the  United  States  Geological  Survey. 


PLATE  I. 

FIG.  1.  OH  vine-bearing-  andesite.  North  Haven. 

Olivine  phenocrysts,  older  than  the  feldspar,  and  now 
altered  to  serpentine.  Magnetite  and  apatite  inclusions 
in  the  olivine. 

FIG.  2.  Andesite.  North  Haven. 

Flow  structure  developed  by  feldspar  laths  of  ground- 
mass. 

FIG.  3.  Augite-andesite.  North  Haven. 

Feldspar  phenocrysts  with  altered  core,  and  other 
crystals  of  labradorite  zonal. 

FIG.  4.  Rhyolitic  tuff,  North  Haven. 

Ash-structure  developed  by  glass  sherds,  now  devitri- 
iied.  Fragment  of  lava  with  quartz  phenocryst. 

FIG.  5.  Aporhyolitic  flow-breccia.  Vinal  Haven. 

Glass  fragment  with  obsidian  structure. 

FIG.  6.  Spherulitic  aporhyolite.  Vinal  Haven. 

Spherulitic  intergrowth  partially  altered  to  a  granular 
mosaic.  Nicols  crossed. 


PLATE    I 


X   22   FIG.   I. 


X   I  34  FIG.  2. 


X   22   FIG.  3. 


X  44  FIG.  4. 


X    22O  FIG.   5. 


X  44   FIG.  6. 


i  i  11  ii  m  a 


YD   ,'0053 


